pgsql: doc: Fix link to TCL project

doc: Fix link to TCL project

The TCL project previously used the domain tcl.tk, but started to
redirect traffic to tcl-lang.org some time ago. Since 2026-10-31
the old tcl.tk domain is hosting a site for Android apps with no
connection to the TCL project. Update all existing tcl.tk links
with their corresponding tcl-lang.org url.

Backpatch to all supported versions.

Author: Daniel Gustafsson <daniel(at)yesql(dot)se>
Reported-by: Joshua Harris <joshuaharris(at)salesforce(dot)com>
Discussion: https://postgr.es/m/179450008042.706202.15568895130164005440@wrigleys.postgresql.org
Backpatch-through: 15

Branch
------
REL_17_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/999b85bbb5513fdfabd2efa4c4adf8e832056aad

Modified Files
--------------
doc/src/sgml/pltcl.sgml | 2 +-
src/test/regress/expected/regex.out | 2 +-
src/test/regress/sql/regex.sql | 2 +-
3 files changed, 3 insertions(+), 3 deletions(-)