[Docs] Updated instances.rst, fixing a typo in instances grammar

The documentation page on instances (https://gitlab.haskell.org/ghc/ghc/-/blob/master/docs/users_guide/exts/instances.rst) gives BNF-style grammar for the tops of instance declarations. I think there is a typo there (see below) and this PR addresses for it.

Changes proposed:-

from

arg_type ::= <empty>
          |  arg_type arg_types

to

arg_types ::= <empty>
           |  arg_type arg_types