Meta: Update spec to Stage 4 (#51)

Author: gibson042

spec.html

@@ -4,7 +4,7 @@
 <pre class="metadata">
 title: JSON.parse source text access
 status: proposal
-stage: 3
+stage: 4
 shortname: &lt;a href="https://github.com/tc39/proposal-json-parse-with-source"&gt;proposal-json-parse-with-source&lt;/a&gt;
 contributors: Richard Gibson
 </pre>