doc: globally change CRUSH ruleset to CRUSH rule

doc: globally change CRUSH ruleset to CRUSH rule

Since kraken, Ceph enforces a 1:1 correspondence between CRUSH ruleset and
CRUSH rule, so effectively ruleset and rule are the same thing, although
the term "ruleset" still survives - notably in the CRUSH rule itself, where it
effectively denotes the number of the rule.

This commit updates the documentation to more faithfully reflect the current
state of the code.

Fixes: http://tracker.ceph.com/issues/20559
Signed-off-by: Nathan Cutler <ncutler@suse.com>