HPCC-11349 DOCS:DTD Errors

Fix HPCC-11349 DOCS:DTD Errors
Fix for couple DTD errors reported
on remote build machine.
dtd needs to declare as docbook 4.5

Signed-off-by: g-pan <greg.panagiotatos@lexisnexis.com>
Merge pull request #5813 from g-pan/H11349-IMDB-DTD

HPCC-11349 DOCS:DTD Errors

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>