Fix: Fix a syntax error

```
Fatal error: Array and string offset access syntax with curly braces is no longer supported in Standard input code on line 124
Errors parsing Standard input code
```

Author: michaeluno

example/taxonomy_field/APF_TaxonomyField.php

@@ -121,7 +121,7 @@ public function do_APF_TaxonomyField() { // do_{instantiated class name}
      */
     public function replyToSortCustomColumn( $aTerms, $aTaxonomies, $aArgs ) {
 
-        if ( 'edit-tags.php' == $GLOBALS['pagenow'] && isset( $_GET{'orderby'} ) && 'custom' == $_GET{'orderby'} ) {
+        if ( 'edit-tags.php' == $GLOBALS['pagenow'] && isset( $_GET[ 'orderby' ] ) && 'custom' == $_GET[ 'orderby' ] ) {
             usort( $aTerms, array( $this, '_replyToSortByCustomOptionValue' ) );
         }
         return $aTerms;