wrap_comments=true can add a space to already properly formatted comments

[ahl]

// rustfmt-wrap_comments: true

/// ```xxx```
///
/// xxx
pub struct X;

$ rustfmt --check --config=wrap_comments=true <tests/source/issue-xxx.rs
Diff in <stdin>:1:
 // rustfmt-wrap_comments: true

 /// ```xxx```
-///
+///
 /// xxx
 pub struct X;

error[internal]: left behind trailing whitespace
 --> <stdin>:4:4:4
  |
4 | ///
  |    ^
  |
  = note: set `error_on_unformatted = false` to suppress the warning against comments or string literals

warning: rustfmt has failed to format. See previous 1 errors.

[ytmimi]

I think this might be related to #5421 or even a duplicate of that issue. My guess is that ```xxx``` is incorrectly getting interpreted as the start of a markdown code block.