fix format hint in testing fonts stylesheet

layout/reftests/fonts/sil/silfonts.css

@@ -30,5 +30,5 @@
 
 @font-face {
   font-family: Scheherazade;
-  src: url(ScheherazadeRegAAT.ttf) format(truetype-aat), url(ScheherazadeRegOT.ttf) format(opentype);
+  src: url(ScheherazadeRegAAT.ttf) format("truetype-aat"), url(ScheherazadeRegOT.ttf) format("opentype");
 }