bpo-22295: use python -m pip rather than plain pip in more examples (GH-24003)

Author: scop

docs/using.rst

@@ -40,8 +40,8 @@ something into it::
 
     $ python3 -m venv example
     $ source example/bin/activate
-    (example) $ pip install importlib_metadata
-    (example) $ pip install wheel
+    (example) $ python -m pip install importlib_metadata
+    (example) $ python -m pip install wheel
 
 You can get the version string for ``wheel`` by running the following::