"Number of keys (estimate)" in the output of the `nodetool tablestats` should be "Number of parititions (estimate)"

The output of the nodetool tablestats command is ambiguous in regards to the number of keys/partitions. It shows:

Number of keys (estimate): 6

And based on that it is not clear if it is number of primary keys (rows) or number of partition keys.

The fix is trivial.