Use explicit external references

Author: Viicos

docs/source/plugin-development/registering-plugins.rst

@@ -20,6 +20,7 @@ metadata.
 Let's presume that we already have our plugin written and it's in a module
 :external+packaging:term:`Build Backend`, but be aware that most backends
 called ``flake8_example``. We will also assume ``setuptools`` is used as a
+:external+packaging:term:`Build Backend`, but be aware that most backends
 support entry points.
 
 We might have a ``setup.py`` that looks something like: