trivial wrong(old) comment

[scabug]

https://github.com/scala/scala/blob/cbac99417f58e66f257af33951f5c3341d7221a6/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala#L1474

By spec(2.9), correct PrefixExpr is

bq. PrefixExpr ::= [‘-’ | ‘+’ | ‘~’ | ‘!’] SimpleExpr

'&' was added by below commit
scala/scala@fde7c4c

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-6856?orig=1
Reporter: Shinya Mochizuki (lyrical_logical)
Affected Versions: 2.11.4

[scabug]

@retronym said:
Reassinging to 2.11.5, 2.11.4 has already been released.

[scabug]

@retronym said:
scala/scala#4106