[tf] Emit TfDebugSymbolEnableChangedNotice when the debug flag value changes

Previously, this would emit the "symbols changed" which is meant to
notify when the list of available debug flags has changed.

(Internal change: 2361025)

Author: mrawde

pxr/base/tf/debug.cpp

@@ -207,7 +207,7 @@ class ARCH_HIDDEN Tf_DebugSymbolRegistry {
 
         if (changed && _initialized) {
             lock.release();
-            TfDebugSymbolsChangedNotice().Send();
+            TfDebugSymbolEnableChangedNotice().Send();
         }
     }