Update hash_example.py

As input and hash are inbuilt functions, so I'd suggest using other names for variables.
here is output from python interpreter.

>>> hash
<built-in function hash>
>>> input
<built-in function input>

code/hash_example.py

@@ -9,10 +9,10 @@ text = "I am Satoshi Nakamoto"
 for nonce in xrange(20): 
     
     # add the nonce to the end of the text
-    input = text + str(nonce) 
+    input_data = text + str(nonce) 
      
     # calculate the SHA-256 hash of the input (text+nonce)
-    hash = hashlib.sha256(input).hexdigest() 
+    hash_data = hashlib.sha256(input_data).hexdigest() 
     
     # show the input and hash result
-    print input, '=>',  hash 
+    print input_data, '=>',  hash_data