pgsql: Consistently use "superuser" instead of "super user"

Consistently use "superuser" instead of "super user"

The correct nomenclature for the highest privileged user is superuser
and not "super user", this replaces the few instances where that was
used erroneously. No user-visible changes are done as all changes are
in comments, so no back-patching.

Author: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Discussion: https://postgr.es/m/CALj2ACW3snGBD8BAQiArMDS1Y43LuX3ymwO+N8aUg1Hrv6hYNw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f7c53bb9e309cfea3206f4ed44b38eb2bd1cf726

Modified Files
--------------
src/backend/commands/amcmds.c            | 2 +-
src/backend/commands/foreigncmds.c       | 4 ++--
src/backend/commands/tablespace.c        | 2 +-
src/test/regress/expected/conversion.out | 2 +-
src/test/regress/sql/conversion.sql      | 2 +-
5 files changed, 6 insertions(+), 6 deletions(-)