Fix a stray piece of remarkup syntax in arc documentation

Summary: This is incorrectly starting an ordered sublist.

Test Plan: (o_O)

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D17434

src/docs/user/userguide/arcanist_commit_ranges.diviner

@@ -100,7 +100,7 @@ is found. Valid rules are listed below. In this list, "*" means "any string".
      copy parent. (hg-only)
   - `arc:exec(*)` Execute the specified command. The command should determine
     the base revision to use and print it to stdout, then exit with return code
-    0. If the command exits with another return code, the rule will fail. The
+    `0`. If the command exits with another return code, the rule will fail. The
     command will be executed with the root directory of the working copy as the
     current working directory.
   - `arc:bookmark` Use the most recent non-outgoing ancestor of the working