Correct a typo in a declared token in the reference grammar

This appears to not have too much of a detrimental effect, but it
doesn't seem to be what is intended either.

antlr doesn't mind that `PLUS` isn't declared in `tokens` and happily
uses the `PLUS` that appears later in the file, but the generated
RustLexer.tokens had PLUS at the end rather than where it was intended:

NOT=10
TILDE=11
PLUT=12
MINUS=13
...
PLUS=56

0 files changed, 0 insertions, 0 deletions