Replace "can not" with "cannot" in docs, user messages, and tests

test/functional/rpc_decodescript.py

@@ -52,7 +52,7 @@ class DecodeScriptTest(BitcoinTestFramework):
         rpc_result = self.nodes[0].decodescript('5100')
         assert_equal('1 0', rpc_result['asm'])
 
-        # null data scriptSig - no such thing because null data scripts can not be spent.
+        # null data scriptSig - no such thing because null data scripts cannot be spent.
         # thus, no test case for that standard transaction type is here.
 
     def decodescript_script_pub_key(self):