doc: Fix initdb option xreflabels

doc: Fix initdb option xreflabels

Generally, we don't want any overriding xreflabels in the options
list, so that we can link to options and the link renders as the
option name.  The -g option did this differently and config.sgml made
use of that for a link.  The new --no-data-checksums option (commit
983a588e0b8) apparently copied this pattern, but that seems like the
wrong direction, as a future patch revealed.

To fix, remove the two xreflabels and rewrite the link in config.sgml
with an explicit link text.