Spelling fixes for the docs

Some explanation [here](https://owl.english.purdue.edu/owl/resource/591/01/)

> When "u" makes the same sound as the "y" in "you," or "o" makes the same sound as "w" in "won," then a is used. The word-initial "y" sound ("unicorn") is actually a glide [j] phonetically, which has consonantal properties; consequently, it is treated as a consonant, requiring "a."