Docs: ensure inclusion of jp2 function

A misplaced code coverage comment was preventing this.
See ef849fd for the original commit where this broke.

lib/output.js

@@ -632,6 +632,7 @@ function gif (options) {
   return this._updateFormatOut('gif', options);
 }
 
+/* istanbul ignore next */
 /**
  * Use these JP2 options for output image.
  *
@@ -665,7 +666,6 @@ function gif (options) {
  * @returns {Sharp}
  * @throws {Error} Invalid options
  */
-/* istanbul ignore next */
 function jp2 (options) {
   if (!this.constructor.format.jp2k.output.buffer) {
     throw errJp2Save();