update web3-shh.rst : whisper-overview broken link (#4170)

whisper overview has been moved from https://github.com/ethereum/go-ethereum/wiki/Whisper to https://eth.wiki/concepts/whisper/whisper-overview

Author: starwalker00

docs/web3-shh.rst

@@ -5,7 +5,7 @@ web3.shh
 ========
 
 
-The ``web3-shh`` package allows you to interact with the whisper protocol for broadcasting. For more see `Whisper Overview <https://github.com/ethereum/go-ethereum/wiki/Whisper>`_.
+The ``web3-shh`` package allows you to interact with the whisper protocol for broadcasting. For more see `Whisper Overview <https://eth.wiki/concepts/whisper/whisper-overview>`_.
 
 
 .. code-block:: javascript