updated code that sliced dictionary keys, for Python 3 compatibility, c…

…loses #69

book/ch04.rst

@@ -1518,7 +1518,7 @@ progress if a ``verbose`` flag is set:
     ...             freqdist[word] += 1
     ...             if verbose and freqdist.N() % 100 == 0: print(".")
     ...     if verbose: print
-    ...     return freqdist.keys()[:num]
+    ...     return freqdist.most_common(num)
 
 .. caution::
    Take care not to use a mutable object as the default value of