Fix typo in doc hyperlink [skip appveyor]

A reference to opt-help was missing the "help" part.

Signed-off-by: Mats Wichmann <mats@linux.com>

Author: mwichmann

SCons/Script/SConscript.xml

@@ -278,7 +278,7 @@ file is found.
 <para>
 Adds <parameter>text</parameter> to the help message shown when
 &scons; is called with the
-<link linkend="opt-"><option>-h</option> or <option>--help</option></link>
+<link linkend="opt-help"><option>-h</option> or <option>--help</option></link>
 argument.
 </para>
 <para>