SubmittingPatches: call gitk's command "Copy commit reference"

Documentation/SubmittingPatches informs the contributor that gitk's
context menu command "Copy commit summary" can be used to obtain the
conventional format of referencing existing commits.  This command in
gitk was renamed to "Copy commit reference" in commit [1], following
implementation of Git's "reference" pretty format in [2].

Update mention of this gitk command in Documentation/SubmittingPatches
to its new name.

[1] b8b6095 (gitk: rename "commit summary" to "commit reference",
    2019-12-12)
[2] commit 1f0fc1d (pretty: implement 'reference' format, 2019-11-20)

Signed-off-by: Andrei Rybak <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rybak

Documentation/SubmittingPatches

@@ -266,7 +266,7 @@ date)", like this:
 	noticed that ...
 ....
 
-The "Copy commit summary" command of gitk can be used to obtain this
+The "Copy commit reference" command of gitk can be used to obtain this
 format (with the subject enclosed in a pair of double-quotes), or this
 invocation of `git show`: