Example for installation of multiple extras

Documention did not have an example for installation of multiple extras.
To the reader it is not clear if SomePackage[extra1,extra2] works or
SomePackage[extra1] SomePackage[extra2].

(Spoiler alert: Donald told me, it's SomePackage[extra1,extra2].)
Signed-off-by: NChristian Heimes <christian@python.org>

docs/reference/pip_install.rst

@@ -801,6 +801,7 @@ Examples
       $ pip install git+https://git.repo/some_pkg.git#egg=SomePackage[PDF]
       $ pip install SomePackage[PDF]==3.0
       $ pip install -e .[PDF]==3.0  # editable project in current directory
+      $ pip install SomePackage[PDF,EPUB]  # multiple extras
 
 
 #. Install a particular source archive file.