Fix some minor issues in the guide.

[parir]

This PR fixes some minor grammar and spelling issues in the guide.

[lilyball]

rust is already the default in rustdoc for code blocks.

Additionally, the {} is not necessary for adding a single attribute to a code block.

[parir]

This basically just fixes syntax highlighting if you read it on GitHub.

[lilyball]

Which is rather irrelevant. I don't have any objection to new documentation using

```rust

but I don't think this is a good reason to add unnecessary modifications.

[parir]

Hmm. I just imitated what was already there. "```{rust}" is used all over the place in the guide.

Edit: "```rust" is also used quite frequently. This needs some consistency. :)

[parir]

Should I revert this change?

[steveklabnik]

I actually do want all of them to have the {rust}

[lilyball]

I'll concede to your desires on the subject of guides, though I still maintain that the {} is unnecessary.

[parir]

So what should we do? Leave it as it is and maybe open a seperate PR for this issue?

[parir]

@kballard PTAL.

[lilyball]

LGTM. I just edited your PR description to remove the strikethrough because bors uses that in the merge commit.