coqPackages docs: fix typo (and thus manual build)

Changed files
+1 -1
doc
languages-frameworks
+1 -1
doc/languages-frameworks/coq.xml
···
</para>
<para>
Coq libraries may be compatible with some specific versions of Coq only.
-
The <liberal>compatibleCoqVersions</liberal> attribute is used to
+
The <literal>compatibleCoqVersions</literal> attribute is used to
precisely select those versions of Coq that are compatible with this
derivation.
</para>