PCBC-229: Fix --with-couchbase help text

This option is used to enable the module if the module is located
in "ext" as part of the build process.

Change-Id: If5eb77a4fcc7abb1e28efeb810bf492a9a49e2a2
Reviewed-on: http://review.couchbase.org/26237
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>