Fix spacing in code sample.

doc/development/frontend.md

@@ -227,6 +227,7 @@ class MyThing {
 gl.MyThing = new MyThing();
 // best
+
 let singleton;
 class MyThing {
@@ -246,6 +247,7 @@ class MyThing {
 }
 gl.MyThing = MyThing;
+
 ```
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