fix outdated docs for ParseError

[whispersofthedawn]

The referenced unused variant seems to have been removed in commit 52d736e with the move to #[non_exhaustive].

[codecov]

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[whispersofthedawn]

Hi! I see that there's some sort of test failure to do with sanitizers, but I don't think that has anything to do the changes in this PR. I can't imagine why this error (repeated several times) would be related to this trivial docs change:

error: mixing `-Zsanitizer` will cause an ABI mismatch in crate `data_url`
  |
  = help: the `-Zsanitizer` flag modifies the ABI so Rust crates compiled with different values of this flag cannot be used together safely
  = note: unset `-Zsanitizer` in this crate is incompatible with `-Zsanitizer=thread` in dependency `core`
  = help: set `-Zsanitizer=thread` in this crate or unset `-Zsanitizer` in `core`
  = help: if you are sure this will not cause problems, you may use `-Cunsafe-allow-abi-mismatch=sanitizer` to silence this error

Given that this is a nightly-only error, I assume this is an issue with CI configuration? I suspect it's appearing now as a result of rust-lang/rust#138736.

[Manishearth]

Yes. I don't recall if that test blocks the merge queue, but we may need to fix it before being able to land this.