Fix some typos in strings and comments

[sndth]

This PR fixes several typos found in a few strings as well as in many comments, but it does not affect the code's functionality in any way. There are many more typos, and I will try to fix some more.

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[fktn-k]

@sndth
Thanks for fixing so many typos and gramatical mistakes.
I requested a tiny change, but the others look good to me.

[fktn-k]

Looks good to me.

[fktn-k]

@sndth
Let me know when you're done with all fixes.

[sndth]

Currently that's all!

[fktn-k]

@sndth
OK. Thanks a lot!
I'll merge the changes.