rust: doctest: fix incorrect pattern in replacement

rust: doctest: fix incorrect pattern in replacement

The `-> Result<(), impl core::fmt::Debug>` string is generated by rustdoc
and by adding "::" into the string it no longer finds anything, making
the line useless.

Remove the "::" in the pattern. Omit it in the replacement too, for
consistency with upstream rustdoc.

Fixes: de7cd3e4d638 ("rust: use absolute paths in macros referencing core and kernel")
Signed-off-by: Gary Guo <gary@garyguo.net>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260616132559.2245814-1-gary@kernel.org
[ Added link in code comment to `rustdoc`'s 1.87 PR that fully qualified
  it for context. Improved comments for consistency. Reworded to drop
  changelog and to fix typo. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>