Correct warning condition (#2049)

Summary:
**Summary**

To my knowledge, ```warning``` works the way ```invariant``` does; it warns if the first argument is false not true.

Prior to this PR, ```moveSelectionBackward``` warns when the selection is collapsed when it should do the opposite.
Pull Request resolved: #2049

Differential Revision: D14699712

fbshipit-source-id: bd649fa6fdc60346d80be27ab4217324c3ef21d6

Author: scrollbars

src/component/handlers/edit/commands/moveSelectionBackward.js

@@ -31,7 +31,7 @@ function moveSelectionBackward(
   const selection = editorState.getSelection();
   // Should eventually make this an invariant
   warning(
-    !selection.isCollapsed(),
+    selection.isCollapsed(),
     'moveSelectionBackward should only be called with a collapsed SelectionState',
   );
   const content = editorState.getCurrentContent();

src/component/handlers/edit/commands/moveSelectionForward.js

@@ -31,7 +31,7 @@ function moveSelectionForward(
   const selection = editorState.getSelection();
   // Should eventually make this an invariant
   warning(
-    !selection.isCollapsed(),
+    selection.isCollapsed(),
     'moveSelectionForward should only be called with a collapsed SelectionState',
   );
   const key = selection.getStartKey();