Fix a minor grammar error (#636)

Author: davet2001

Diff for: docs/plugins.rst

@@ -2,7 +2,7 @@
 Plugin coverage
 ===============
 
-Getting coverage on pytest plugins is a very particular situation. Because how pytest implements plugins (using setuptools
+Getting coverage on pytest plugins is a very particular situation. Because of how pytest implements plugins (using setuptools
 entrypoints) it doesn't allow controlling the order in which the plugins load.
 See `pytest/issues/935 <https://github.com/pytest-dev/pytest/issues/935#issuecomment-245107960>`_ for technical details.