[1.5.x] Fixed #19683 - Added a missing import in signing example.

Thanks sunsongxp@ for the report.

Backport of 537d44b1b9 from master

docs/topics/signing.txt

@@ -60,6 +60,7 @@ You can retrieve the original value using the ``unsign`` method::
 If the signature or value have been altered in any way, a
 ``django.core.signing.BadSignature`` exception will be raised::
 
+    >>> from django.core import signing
     >>> value += 'm'
     >>> try:
     ...    original = signer.unsign(value)