s/command-line/command line/g

A follow-up to:

commit 3f85ef0f05ffc51e19f86fb83a1c51e8e3cd6817
Author: Harald Hoyer <harald@redhat.com>
Date:   Thu Nov 6 15:33:48 2014 +0100

    s/commandline/command line/g
diff --git a/tools/make-directive-index.py b/tools/make-directive-index.py
index 5c80a7b..17b1325 100755
--- a/tools/make-directive-index.py
+++ b/tools/make-directive-index.py
@@ -141,7 +141,7 @@
         </refsect1>
 
         <refsect1>
-                <title>command-line options</title>
+                <title>command line options</title>
 
                 <para>Command-line options accepted by programs in the
                 systemd suite.</para>