It's ok! We all do it. We also tend to read 2-3 letter words quickly and that is why you missed it; rest assured that some of your app readers will, too.
If it helps make you feel better, I co-authored a book several years ago. I combed through that thing a zillion times. We had publishing and marketing folks on it. Yet, on p. 81, there is a @"$&!?! typo. A 2-letter word missing, to be exact. Argh!! I learned a lot from that experience.
If you're not done or there are others reading this who have yet to submit their statements, I'd like to share my process FWIW:
For my 9 unique PhD statements, I had 3 levels of readers after I had finished a (what I thought was) solid SOP.
Reader 1 (2 apps per reader): general feedback, confusions and clarifications needed, proofreading.
I did serious revisions, then,
Reader 2 (4-5 apps per reader): proofreading only.
I was really surprised by how many errors they found. I revised it again. I also read it OUT LOUD to listen for flow, and revised for that. I highly recommend doing this part if you have not. You'll be surprised by what you catch when you read your work out loud! Finally, just before I submitted it, I sent it to:
Reader 3 (one person): final proofreading only (no content comments, please).
Reader 3 caught a few missing words and simple typos, as well as looked up what I should have in my headers (i.e., PhD vs Ph.D., which varied by program, and not something I caught or thought of on my own).
This whole process took a week and involved 10 people. I created a timeline and deadlines for each reader. Reader 1 was given 2 days, reader 2 had 2 days, and reader 3 just 1 day. I asked their permission ahead of time and explained which part of the assembly line they were in ?
Finally, I converted the final SOP Word doc into a PDF and checked it one last time before I submitted it. This is important because sometimes the formatting was lost/changed when uploaded into the online applications as a Word doc. I noticed this with my CV as well, so while I'm pretty sure one tiny formatting error wouldn't disqualify us from acceptance, I do believe presentation matters.
This system worked well and I feel really good about the way my statements turned out. I haven't gotten any official acceptances yet, but I do have an unofficial acceptance and an interview coming up.
For readers, I asked my willing Dr. friends to edit my top choices, and then asked friends who'd finished a master's or were great readers. Everyone was happy to help!
I have already earned two graduate degrees (MA, Ed.M.), am a writing teacher, published technical author, fiction writer, and I earned a 6.0 on the GRE writing, and yet I still make careless errors. I've just learned that this is something I do, so I need help. I feel all of those experiences really helped me structure this SOP writing journey and application process, FWIW.
I hope this process helps someone going forward. Good luck everyone!