[linalg.conj.conjugatedaccessor] Fix incorrect return type of nested_accessor

[eisenwave]

One part of #7214.

As @mhoemmen explains:

Fix return type of nested_accessor member function

The nested_accessor member function of conjugated_accessor (which is defined inline in the class' synopsis) currently returns const Accessor&. This is incorrect, because there is no Accessor type in scope.

The intent is for the function to return const NestedAccessor&. As main author of this proposal, this is the intent. At some point in revisions, the template parameter was renamed from Accessor to NestedAccessor. It's likely that we missed changing the return type of nested_accessor.

@jwakely does this need an LWG issue or can we "just fix this"?

[jwakely]

Seems editorial