docs: avoid repeating the color

The color attribute is inherited in CSS, so there's no point in repeating
this.

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

docs/mesa.css

@@ -8,19 +8,16 @@ body {
 h1 {
 	font-size: 24px;
 	font-weight: bold;
-	color: black;
 }
 
 h2 {
 	font-size: 18px;
 	font-weight: bold;
-	color: black;
 }
 
 code {
 	font-family: monospace;
 	font-size: 10pt;
-	color: black;
 }