docs: Conform to coding style in HACKING guide

The document is supposed to outline our best practices on the
javascript side, so make sure the code snippets actually conform
to the enforced style.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/919

HACKING.md

@@ -29,9 +29,8 @@ what to do.
             bar = do_thing(b);
 
         if (var == 5) {
-            for (let i = 0; i < 10; i++) {
+            for (let i = 0; i < 10; i++)
                 print(i);
-            }
         } else {
             print(20);
         }
@@ -102,9 +101,8 @@ under the imports:
 Always use either `const` or `let` when defining a variable.
 ```javascript
     // Iterating over an array
-    for (let i = 0; i < arr.length; ++i) {
+    for (let i = 0; i < arr.length; ++i)
         let item = arr[i];
-    }
 
     // Iterating over an object's properties
     for (let prop in someobj) {
@@ -252,7 +250,7 @@ variable that can be captured in closures.
 All closures should be wrapped with Function.prototype.bind or use arrow
 notation.
 ```javascript
-    let closure1 = () => { this._fnorbate(); };
+    let closure1 = () => this._fnorbate();
     let closure2 = this._fnorbate.bind(this);
 ```