Fix typos in atomic_ref test names

[frasercrmck]

I'm assuming these are typos.

There are some inconsistencies about how we style the test names for ostensibly the same operations: see tests covering one operator (atomic_ref::operator) vs those that cover two (atomic_ref operator/operator - no colons). I see why it was done, but the inconsistency is noticeable.

[keryell]

This is why we need more code introspection in future C++ to avoid writing such boilerplate description manually. :-)