docs: fix default font-size=12 documentation

We used font-size=15 in the past but changed it long ago. However, the
documentation still used 15 so fix this now.

Reported-by: James Buren <ryu0@ymail.com>
Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>

docs/man/kmscon.xml

@@ -478,7 +478,7 @@
       <varlistentry>
         <term><option>--font-size {points}</option></term>
         <listitem>
-          <para>Font size in points. (default: 15)</para>
+          <para>Font size in points. (default: 12)</para>
         </listitem>
       </varlistentry>
 

src/kmscon_conf.c

@@ -141,7 +141,7 @@ static void print_help()
 		"Font Options:\n"
 		"\t    --font-engine <engine>  [pango]\n"
 		"\t                              Font engine\n"
-		"\t    --font-size <points>    [15]\n"
+		"\t    --font-size <points>    [12]\n"
 		"\t                              Font size in points\n"
 		"\t    --font-name <name>      [monospace]\n"
 		"\t                              Font name\n"