git fetch documentation: describe short '-p' synonym to '--prune' option

It's already implemented, just undocumented.

Signed-off-by: Oren Held <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: orenhe

Documentation/fetch-options.txt

@@ -34,6 +34,7 @@ ifndef::git-pull[]
 	Allow several <repository> and <group> arguments to be
 	specified. No <refspec>s may be specified.
 
+-p::
 --prune::
 	After fetching, remove any remote tracking branches which
 	no longer exist	on the remote.