docs: add missing semicolon

While it's legal to omit the last semicolon in a CSS block, it's
generally not considered good style, as it makes it harder to add new
lines.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

docs/mesa.css

@@ -42,7 +42,7 @@ iframe {
 	position: absolute;
 	left: 20em;
 	right: 10px;
-	overflow: hidden
+	overflow: hidden;
 }
 
 .header {