xhtml export: Rather use rem than cm for margin

follow-up for 46b4eb8b

Change-Id: I70986537b842b7fd26de9c2efd1fbdad7b5271a1
Reviewed-on: https://gerrit.libreoffice.org/41990Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>

filter/source/xslt/odf2xhtml/export/xhtml/body.xsl

@@ -1282,7 +1282,7 @@
 				<!-- Add some margin (a tab is around 0.64cm in LO), but only if there is a number preceding the heading. -->
 				<xsl:if test="$labelFollowedBy='listtab' and $headingNumber != ''">
 					<xsl:attribute name="style">
-						<xsl:text>margin-right:0.64cm;</xsl:text>
+						<xsl:text>margin-right:0.8rem;</xsl:text>
 					</xsl:attribute>
 				</xsl:if>
 				<xsl:copy-of select="$headingNumber"/>