Fix mis-spelling of aria-describedby

This is currently spelt incorrectly though it
seemed to work nonetheless. Can only assume this
is a common error, for this attribute or all
attribute names, so browsers work it out.

This makes the spelling match the spec:

https://www.w3.org/TR/wai-aria/#aria-describedby

app/assets/javascripts/updateStatus.js

@@ -49,11 +49,11 @@
 
       this.$textbox
         .attr(
-          'aria-described-by',
+          'aria-describedby',
           (
-            this.$textbox.attr('aria-described-by') || ''
+            this.$textbox.attr('aria-describedby') || ''
           ) + (
-            this.$textbox.attr('aria-described-by') ? ' ' : ''
+            this.$textbox.attr('aria-describedby') ? ' ' : ''
           ) + id
         )
         .on('input', throttle(this.update, 150))