Fix the description of raw byte string literals
auto merge of #18052 : IvanUkhov/rust/raw-byte-string-literals, r=aturon

Hello,


The pull request fixes a typo in the description of raw string literals in [reference.md](https://github.com/rust-lang/rust/blob/master/src/doc/reference.md#byte-and-byte-string-literals).


Regards,
Ivan