Fix entry point key typo in docs (#2373)

Author: uranusjr

docs/extend.rst

@@ -37,7 +37,7 @@ Creators are what actually perform the creation of a virtual environment. The bu
 all achieve this by referencing a global install; but would be just as valid for a creator to install a brand new
 entire python under the target path; or one could add additional creators that can create virtual environments for other
 python implementations, such as IronPython. They must be registered under and entry point with key
-``virtualenv.discovery`` , and the class must implement :class:`virtualenv.create.creator.Creator`:
+``virtualenv.create`` , and the class must implement :class:`virtualenv.create.creator.Creator`:
 
 .. code-block:: ini