doc: fix git grep args order in Quick Reference

The example provided has its arguments in the wrong order. The revision
should follow the pattern, and not the other way around.

Signed-off-by: Guillaume Jacob <guillaume@absolut-sensing.com>

Author: guillaume-jcb

Documentation/user-manual.adoc

@@ -4466,7 +4466,7 @@ $ git show		    # most recent commit
 $ git diff v2.6.15..v2.6.16 # diff between two tagged versions
 $ git diff v2.6.15..HEAD    # diff with current head
 $ git grep "foo()"	    # search working directory for "foo()"
-$ git grep v2.6.15 "foo()"  # search old tree for "foo()"
+$ git grep "foo()" v2.6.15  # search old tree for "foo()"
 $ git show v2.6.15:a.txt    # look at old version of a.txt
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