bindings: standardize link format

[jmayclin]

Problem:

Items linked in rust doc comments are inconsistently formatted.

Discussed in #5061 (comment).

The most common structure is

[s2n_api]

But we also use

[`s2n_api`]

which renders the links in a monospaced font.

Solution:

We should standardize on the monospaced approach.

[`s2n_api`]

This seems to be the standard, as seen here in tokio docs, and as seen here in the rustdoc book.

Requirements / Acceptance Criteria:

• all links to code items use the monospace format
• grep_simple_mistakes is updated to account for the new format, and warn on the old one.

[manojks1999]

@jmayclin can I work on this?

[jmayclin]

sure!