Fix heading markup in hot_spec.rst

The Resource Dependencies section in hot_spec.rst used a wrong
heading markup (inconsistent with previous sections' 2nd level
markups), so the heading did not appear in the HTML.

This patch fixes the markup.

Change-Id: Id10ca0246942b2b64ea6269413243379c3ae2158
Closes-Bug: #1295538
Merge "Fix heading markup in hot_spec.rst"