Fix spelling mistakes/typos in SleeveDecreases.md

[Terri Oda]

I forgot to run a spell check on https://github.com/terriko/pyknit/blob/main/documentation/SleeveDecreases.md and almost certainly left a few typos in. If someone wants an easy first contribution, run a spell check on that file and fix it up!

[Radhika]

@terriko Can I work on it?

[Terri Oda]

Please go ahead! Hopefully it won't be too typo ridden.

[Terri Oda]

Fixed in #7