Fix docs badge

Author: nicoddemus

README.rst

@@ -25,7 +25,7 @@ Besides undoing the mocking automatically after the end of the test, it also pro
 nice utilities such as ``spy`` and ``stub``, and uses pytest introspection when
 comparing calls.
 
-|python| |version| |anaconda| |ci| |coverage| |black| |pre-commit|
+|python| |version| |anaconda| |docs| |ci| |coverage| |black| |pre-commit| 
 
 .. |version| image:: http://img.shields.io/pypi/v/pytest-mock.svg
   :target: https://pypi.python.org/pypi/pytest-mock
@@ -48,7 +48,7 @@ comparing calls.
 .. |pre-commit| image:: https://results.pre-commit.ci/badge/github/pytest-dev/pytest-mock/master.svg
    :target: https://results.pre-commit.ci/latest/github/pytest-dev/pytest-mock/master
 
-.. image:: https://readthedocs.org/projects/pytest-mock/badge/?version=latest
+.. |docs| image:: https://readthedocs.org/projects/pytest-mock/badge/?version=latest
    :target: https://pytest-mock.readthedocs.io/en/latest/?badge=latest