Use .source.gfm as root selector

This matches the grammar's scope name and was an oversight from the
original CSS to Less conversion.

static/markdown.less

@@ -1,19 +1,17 @@
-.source {
-  .gfm {
-    .markup.heading {
-      font-weight: bold;
-    }
+.source.gfm {
+  .markup.heading {
+    font-weight: bold;
+  }
 
-    .bold {
-      font-weight: bold;
-    }
+  .bold {
+    font-weight: bold;
+  }
 
-    .italic {
-      font-style: italic;
-    }
+  .italic {
+    font-style: italic;
+  }
 
-    .comment.quote {
-      font-style: italic;
-    }
+  .comment.quote {
+    font-style: italic;
   }
 }