Fix css documentation typo for color definition

'ligher' should be 'lighter', of course.

https://bugzilla.gnome.org/show_bug.cgi?id=773246

docs/reference/gtk/css-overview.xml

@@ -587,7 +587,7 @@ background-color: @bg_color;
       set of base colors.
     </para>
 
-<literallayout><code>〈color expression〉 = ligher(〈color〉) | darker(〈color〉) | shade(〈number〉,〈color〉) | alpha(〈number〉,〈color〉) | mix(〈number〉,〈color〉,〈color〉)</code>
+<literallayout><code>〈color expression〉 = lighter(〈color〉) | darker(〈color〉) | shade(〈number〉,〈color〉) | alpha(〈number〉,〈color〉) | mix(〈number〉,〈color〉,〈color〉)</code>
 </literallayout>
 
     <para>