pgsql: doc: Add missing values for meson options -Dcassert and -Db_cove

doc: Add missing values for meson options -Dcassert and -Db_coverage

The -Dcassert and -Db_coverage that can be specified to a meson command
require values after an equal sign but the documentation did not
properly reflect that.  All the other options specify the argument
values they expect, so close the gap.

Author: Jelte Fennema
Discussion: https://postgr.es/m/CAGECzQRXd1z+AoQ4tC5tqPk1_NQJohf6xwdEL=z9KgxHau2maQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/1a943d03d60e6a45036c77ca31d02ebd6e3ee169

Modified Files
--------------
doc/src/sgml/installation.sgml | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)