git-config.txt: fix example

The "--add" option is required to add a new value to a multivalued
configuration entry.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-config.txt

@@ -326,7 +326,7 @@ To actually match only values with an exclamation mark, you have to
 To add a new proxy, without altering any of the existing ones, use
 
 ------------
-% git config core.gitproxy '"proxy-command" for example.com'
+% git config --add core.gitproxy '"proxy-command" for example.com'
 ------------
 
 An example to use customized color from the configuration in your