git-check-ref-format.txt: fixup documentation

die is not a standard shell function. Use
a different shell code for the example.

Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-check-ref-format.txt

@@ -118,8 +118,8 @@ $ git check-ref-format --branch @{-1}
 * Determine the reference name to use for a new branch:
 +
 ------------
-$ ref=$(git check-ref-format --normalize "refs/heads/$newbranch") ||
-die "we do not like '$newbranch' as a branch name."
+$ ref=$(git check-ref-format --normalize "refs/heads/$newbranch")||
+{ echo "we do not like '$newbranch' as a branch name." >&2 ; exit 1 ; }
 ------------
 
 GIT