docs: Fix short hash link to github

The escaped newline in the middle of the variable reference breaks the
short hash substitution, leaving the link text exmpty; rewrap so that
each replacement is on its own line.
auto merge of #17777 : lambda/rust/fix-docs-short-hash, r=alexcrichton

The escaped newline in the middle of the variable reference breaks the
short hash substitution, leaving the link text exmpty; rewrap so that
each replacement is on its own line.