Fix variable spelling error.

Author: wrxpowered

assets/variables.less

@@ -6,7 +6,7 @@
 @process-description-color: @process-title-color;
 @process-tail-color: #e9e9e9;
 @wait-icon-color: #ccc;
-@wait-title-color: gba(0,0,0,.43);
+@wait-title-color: rgba(0,0,0,.43);
 @wait-description-color: @wait-title-color;
 @wait-tail-color: @process-tail-color;
 @finish-icon-color: @process-icon-color;