Documentation: update graph api example.

As of commit 03300c0 the graph API uses '*' for all nodes including merges.
This updates the example in the documentation to match.

Signed-off-by: Allan Caffee <allan.caffee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-history-graph.txt

@@ -148,22 +148,22 @@ outputting that information, if desired.
 ------------
 *
 *
-M
+*
 |\
 * |
 | | *
 | \ \
 |  \ \
-M-. \ \
+*-. \ \
 |\ \ \ \
 | | * | |
 | | | | | *
 | | | | | *
-| | | | | M
+| | | | | *
 | | | | | |\
 | | | | | | *
 | * | | | | |
-| | | | | M  \
+| | | | | *  \
 | | | | | |\  |
 | | | | * | | |
 | | | | * | | |