Docs: Add xref to mapreduce spec
<title>Prerequisites</title>
<p>The following are required prior to running this program:</p>
<ul id="ul_v5h_bxq_dp">
- <li id="kx142094">You must have your MapReduce job defined in a YAML file. For information
- about the Greenplum MapReduce specification, see the <i>Greenplum Database Reference
- Guide</i>.</li>
+ <li id="kx142094">You must have your MapReduce job defined in a YAML file. See the <xref
+ type="dita" scope="local" href="../../ref_guide/yaml_spec.xml">Greenplum MapReduce
+ Specification</xref> in the <i>Greenplum Database Reference Guide</i> for more
+ information.</li>
<li id="kx141300">You must be a Greenplum Database superuser to run MapReduce jobs written
in untrusted Perl or Python.</li>
<li id="kx141310">You must be a Greenplum Database superuser to run MapReduce jobs with
