bucket capability changed inadvertently with snappy addition

In creating a test for a fix for JVMCBC-510, I also found that one of the bucket capabilities was changed, seemingly incorrectly as part of a typo. "xdcrCheckpointing" became "xdcrxdcrCheckpointing" when adding snappy in this commit:
https://github.com/couchbase/ns_server/commit/a274c6484ac3044300bfa3c5e343a21a34ea857a