bpo-38374: Remove weakref.ReferenceError from docs (GH-18452)

Reflecting changes to the code, removed weakref.ReferenceError from weakref.rst and exceptions.rst.

Issue submitter provided evidence that the `weakref.ReferenceError` alias for `ReferenceError` was removed from the code in 2007.  Working with @gvanrossum at PyCascades CPython sprint we looked at the code and confirmed that `weakref.ReferenceError` was no longer in `weakref.py`.

Based on that analysis I removed references `weakref.ReferenceError` from the two documents where it was still being referenced: `weakref.rst` and `exceptions.rst`. 


https://bugs.python.org/issue38374