mediawiki.special.changeslist.legend: Avoid legend as stairs

Skin Modern has
  dt { margin-bottom: 0.1em }
This generates a legend like a stair.
This change resets the margin to the default for .mw-changeslist-legend.

Change-Id: I3cf0301c1f46cfbb1b9170b18ca4dc52a7a7a903
Merge "mediawiki.special.changeslist.legend: Avoid legend as stairs"