doc: use 'symmetric difference' consistently

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitk.txt

@@ -70,7 +70,7 @@ linkgit:git-rev-list[1] for a complete list.
 
 --left-right::
 
-	Mark which side of a symmetric diff a commit is reachable
+	Mark which side of a symmetric difference a commit is reachable
 	from.  Commits from the left side are prefixed with a `<`
 	symbol and those from the right with a `>` symbol.
 

Documentation/rev-list-options.txt

@@ -225,7 +225,7 @@ excluded from the output.
 
 --left-only::
 --right-only::
-	List only commits on the respective side of a symmetric range,
+	List only commits on the respective side of a symmetric difference,
 	i.e. only those which would be marked `<` resp. `>` by
 	`--left-right`.
 +
@@ -757,7 +757,7 @@ ifdef::git-rev-list[]
 endif::git-rev-list[]
 
 --left-right::
-	Mark which side of a symmetric diff a commit is reachable from.
+	Mark which side of a symmetric difference a commit is reachable from.
 	Commits from the left side are prefixed with `<` and those from
 	the right with `>`.  If combined with `--boundary`, those
 	commits are prefixed with `-`.