doc: Clarify write::Error::IllegalCharacter description

The description contains a literal `\n` sequence, which seems to be
for clarification or to facilitate searching for that sequence, but
it was originally just concatenated to the end of the message. This
parenthesizes it instead.

Author: EliahKagan

gix-ref/src/store/file/log/line.rs

@@ -20,7 +20,7 @@ mod write {
     #[derive(Debug, thiserror::Error)]
     #[allow(missing_docs)]
     enum Error {
-        #[error(r"Messages must not contain newlines\n")]
+        #[error(r"Messages must not contain newlines (\n)")]
         IllegalCharacter,
     }