docs(db engines): Updade DB2 connection prefix

[cabo40]

The connection prefix should be different if the DB2 dialect is different.

[codecov-io]

Codecov Report

Merging #12673 (bd9fda6) into master (d80c2ed) will decrease coverage by 3.44%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #12673      +/-   ##
==========================================
- Coverage   66.78%   63.34%   -3.45%     
==========================================
  Files        1018      486     -532     
  Lines       49792    29982   -19810     
  Branches     4877        0    -4877     
==========================================
- Hits        33255    18991   -14264     
+ Misses      16410    10991    -5419     
+ Partials      127        0     -127     

Flag	Coverage Δ
cypress	?
javascript	?
python	63.34% <ø> (-0.80%)	⬇️

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Impacted Files	Coverage Δ
superset/db_engines/hive.py	0.00% <0.00%> (-85.72%)	⬇️
superset/sql_validators/postgres.py	50.00% <0.00%> (-50.00%)	⬇️
superset/db_engine_specs/hive.py	54.61% <0.00%> (-29.24%)	⬇️
superset/databases/commands/create.py	83.67% <0.00%> (-8.17%)	⬇️
superset/databases/commands/update.py	85.71% <0.00%> (-8.17%)	⬇️
superset/connectors/sqla/models.py	84.31% <0.00%> (-6.28%)	⬇️
superset/db_engine_specs/sqlite.py	90.62% <0.00%> (-6.25%)	⬇️
superset/databases/commands/test_connection.py	84.78% <0.00%> (-4.35%)	⬇️
superset/utils/celery.py	96.42% <0.00%> (-3.58%)	⬇️
superset/models/core.py	85.59% <0.00%> (-3.27%)	⬇️
... and 542 more

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