Fix "$id" that should have been "$anchor"

Looks like we missed one, h/t jimblackler.
Also fix wording as the value of "$anchor" is not a URI reference.

Author: handrews

jsonschema-core.xml

@@ -1710,8 +1710,8 @@
                 </figure>
                 <t>
                     When an implementation encounters the &lt;#/$defs/single&gt; schema,
-                    it resolves the "$id" URI reference against the current base URI to form
-                    &lt;https://example.net/root.json#item&gt;.
+                    it resolves the "$anchor" value as a fragment name against the current
+                    base URI to form &lt;https://example.net/root.json#item&gt;.
                 </t>
                 <t>
                     When an implementation then looks inside the &lt;#/items&gt; schema, it