core-tutorial.txt: escape asterisk

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/core-tutorial.txt

@@ -971,7 +971,7 @@ $ git show-branch --topo-order master mybranch
 The first two lines indicate that it is showing the two branches
 and the first line of the commit log message from their
 top-of-the-tree commits, you are currently on `master` branch
-(notice the asterisk `*` character), and the first column for
+(notice the asterisk `\*` character), and the first column for
 the later output lines is used to show commits contained in the
 `master` branch, and the second column for the `mybranch`
 branch. Three commits are shown along with their log messages.