pgsql: meson: Fix doc installation path computation

meson: Fix doc installation path computation

This applies the logic from 8f6858064b (append "postgresql" to some
installation paths if it does not already contain "pgsql" or
"postgres") also to the doc installation directory.

Reviewed-by: Tristan Partin <tristan@neon.tech>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/b018b577-38a2-49c6-8727-adfb577de317@eisentraut.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/7b8399819b9b1a409b11ebfb4027aed56747f923

Modified Files
--------------
meson.build | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)