Fix error message when trait method ends with wrong token

When parsing a trait function, the function must end with either `;` or
`{` (signifying a default implementation). The error message incorrectly
stated that it must be `;` or `}`.

Fixes #6610.
auto merge of #8547 : kballard/rust/trait-parse-err-msg, r=alexcrichton

When parsing a trait function, the function must end with either `;` or
`{` (signifying a default implementation). The error message incorrectly
stated that it must be `;` or `}`.

Fixes #6610.