doc: make the heading consistent

All the other headings are in level 3. This patch fixes the offending
heading to use level 3.

PR-URL: https://github.com/nodejs/node/pull/11569

Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/deprecations.md

@@ -524,7 +524,7 @@ of V8 5.8. It is replaced by Inspector which is activated with `--inspect`
 instead.
 
 <a id="DEP0063"></a>
-#### DEP0063: ServerResponse.prototype.writeHeader()
+### DEP0063: ServerResponse.prototype.writeHeader()
 
 Type: Documentation-only