Escape backslash in single_char_pattern.rs

[frobiac]

Escape backslash in single_char_pattern.

Previously, the proposed clippy fix for a single backslash in raw strings r"\" or r#"\"# was also only an unescaped, single '\':

warning: single-character string constant used as pattern
2 |     let s = r#"abc\xyz/"#.find(r"\");
  |                                ^^^^ help: try using a `char` instead: `'\'`
  |
  = note: `#[warn(clippy::single_char_pattern)]` on by default

This PR corrects this to a properly escaped double backslash '\\'.
I haven't come up with any other problematic cases, a single quote was already handled.

Closes: #8060

changelog: Escape backslash in [`single_char_pattern`]

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @giraffate (or someone else) soon.

Please see the contribution instructions for more information.

[giraffate]

@bors r+

It looks good, thanks!

[bors]

📌 Commit 5cc451b has been approved by giraffate

[bors]

⌛ Testing commit 5cc451b with merge fddef24...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: giraffate
Pushing fddef24 to master...