Fix pre-splitting / chunk-moving example

The example doesn't actually suggest any collection size (certainly not "100 million documents", why would it?).

The example given in the second part is actually wrong -- since it divides by 6 rather than computing a modulus, the index would quickly exceed the array length, rather than round-robinning through it.

Signed-off-by: Sam Kleinman <samk@10gen.com>