git-shortlog.txt: fix example about .mailmap

In the example, Joe Developer has <joe@example.com> as his email,
but in the .mailmap is <joe@random.com>. Use example.com instead.

Signed-off-by: Michele Ballabio <barra_cuda@katamail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-shortlog.txt

@@ -82,7 +82,7 @@ her family name fully spelled out, a proper `.mailmap` file would look like:
 # Note how we don't need an entry for <jane@laptop.(none)>, because the
 # real name of that author is correct already, and coalesced directly.
 Jane Doe <jane@desktop.(none)>
-Joe R. Developer <joe@random.com>
+Joe R. Developer <joe@example.com>
 ------------
 
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