libcore/pin: Minor grammar corrections for module documentation

[laumann]

This is by no means exhaustive, but I noticed a few grammatical errors
when reading the documentation, and decided just to push these.

Some standard rules/guidelines I followed:

• Do not split infinitives, ie "not to move" instead of "to not move"

• Do not use "since" when you want to say "because" or "as" - the word
  "since" has a temporal meaning

In addition:

• Fix a small typo: "Similarily" should be "Similarly"

• Delete double-spaces after full stop

[rust-highfive]

r? @Mark-Simulacrum

(rust_highfive has picked a reviewer for you, use r? to override)

[Mark-Simulacrum]

I don't feel comfortable reviewing this one way or another... randomly reassigning to r? @Centril

[Centril]

Thanks! r=me rollup with changes below.

[laumann]

@Centril My Github foo is not that strong, I have never used suggestions, if I use the UI to accept them, does that mean four individual commits? Would it then be better just to push you suggestions as one commit?

[Centril]

@laumann GitHub has a "Add suggestion to batch" functionality if you want to do a single commit; otherwise doing it manually is fine also. Up to you. :)

[laumann]

OK, will do, thanks!

[Centril]

@bors r+ rollup

[bors]

📌 Commit fb61b85 has been approved by Centril