[fixed] Spelling error

Signed-off-by: Geoff Pleiss <gpleiss@pivotal.io>

Author: August Toman-Yih

lib/assertions.js

@@ -226,7 +226,7 @@ exports.tags = {
 
 exports.render = {
   NO_LABEL: {
-    msg: 'You have an unlabled element or control. Add `aria-label` or `aria-labelledby` attribute, or put some text in the element.',
+    msg: 'You have an unlabeled element or control. Add `aria-label` or `aria-labelled-by` attribute, or put some text in the element.',
     test (tagName, props, children, failureCB) {
       if (isHiddenFromAT(props) || presentationRoles.has(props.role))
         return;