Fix documentation typos

Problem/Motivation

There are some documentation typos in simpleboxediting.js.

In * <h2 class="simple-box-title"></h1>, h1 should be h2.

In // If <h2 class="simple-box-description"> is present in the existing markup, h2 should be div.

Proposed resolution

Fix.

Remaining tasks

Implement.

User interface changes

None.

API changes

None.

Data model changes

None.