Fix typo in element reference

The tests etc happened to work because it fell back on the
non-standard browser behavior of finding the element on the page with
the id of 'editor' when the editor variable did not exist in the
current scope in one case, and it found a variable bound in an outer
scope that happened to be the correct value in the other

src/extras/selection.js

@@ -69,9 +69,9 @@
     }
 
     if (this.selectionBegan < this.pointPosition()) {
-      return editor.slice(this.selectionBegan, this.pointPosition());
+      return this.slice(this.selectionBegan, this.pointPosition());
     } else {
-      return editor.slice(this.pointPosition(), this.selectionBegan);
+      return this.slice(this.pointPosition(), this.selectionBegan);
     }
   };