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

src/grammar/RustLexer.g4

@@ -8,7 +8,7 @@ lexer grammar RustLexer;
 
 
 tokens {
-    EQ, LT, LE, EQEQ, NE, GE, GT, ANDAND, OROR, NOT, TILDE, PLUT,
+    EQ, LT, LE, EQEQ, NE, GE, GT, ANDAND, OROR, NOT, TILDE, PLUS,
     MINUS, STAR, SLASH, PERCENT, CARET, AND, OR, SHL, SHR, BINOP,
     BINOPEQ, AT, DOT, DOTDOT, DOTDOTDOT, COMMA, SEMI, COLON,
     MOD_SEP, RARROW, FAT_ARROW, LPAREN, RPAREN, LBRACKET, RBRACKET,