use python3 -m when installing pipx (#631)

Author: Chad Smith

source/guides/installing-stand-alone-command-line-tools.rst

@@ -24,8 +24,8 @@ packages, and allows you to safely run the program from anywhere.
 
 ::
 
-  $ pip install --user pipx
-  $ pipx ensurepath  # ensures the path of the CLI application directory is on your $PATH
+  $ python3 -m pip install --user pipx
+  $ python3 -m pipx ensurepath  # ensures the path of the CLI application directory is on your $PATH
 
 .. Note:: You may need to restart your terminal for the path updates to take effect.