pgsql: Fix documentation of pgrowlocks using "lock_type" instead of"mo

Fix documentation of pgrowlocks using "lock_type" instead of "modes"

The example used in the documentation is outdated as well.  This is an
oversight from 0ac5ad5, which bumped up pgrowlocks but forgot some bits
of the documentation.

Reported-by: Chris Wilson
Discussion: https://postgr.es/m/153838692816.2950.12001142346234155699@wrigleys.postgresql.org
Backpatch-through: 9.3

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/e9cff306168743e6ff2abdc03e4857ecb9fb4001

Modified Files
--------------
doc/src/sgml/pgrowlocks.sgml | 18 +++++++++---------
1 file changed, 9 insertions(+), 9 deletions(-)