No worries. It can be difficult to catch spelling mistakes in things that you write or read over many times.

Pro-tip: When I don't have the luxury of a spell-checker, I read the paragraph backwards, one word at a time, and I find that it helps to catch most of the common errors.

When you read text normally, your brain reads ahead[1] and skims (or omits) a lot of errors out of your field of view so you miss a lot of easy typos. Reading backwards mixes up your brain just enough that it stops "helping" you read.

[1] http://www.mrc-cbu.cam.ac.uk/people/matt.davis/Cmabrigde/