Fix "$ref" quoting in section header.

I noticed while adding $comment that keywords are always quoted
in section headings unless the keyword is the entire heading.
This is true across all three specifications, so this trivial
change brings the only outlier in line with everything else.

Author: handrews

jsonschema-core.xml

@@ -390,7 +390,7 @@
             </t>
         </section>
 
-        <section title="Schema references with $ref">
+        <section title='Schema references with "$ref"'>
             <t>
                 The "$ref" keyword is used to reference a schema, and provides the ability to
                 validate recursive structures through self-reference.