docs: fix html end/start tags

for more well-formed html

Signed-off-by: Brian Paul <brianp@vmware.com>

docs/mangling.html

@@ -21,10 +21,10 @@ This results in all the Mesa functions being prefixed with
 To do this, recompile Mesa with the compiler flag -DUSE_MGL_NAMESPACE.
 Add the flag to CFLAGS in the configuration file which you want to use.
 For example:
+</p>
 <pre>
 CFLAGS += -DUSE_MGL_NAMESPACE
 </pre>
-</p>
 
 
 </body>