issue/PR templates: use h2 instead of h3

`h2` is the semantically correct heading here, as `h1` is for 
document titles and `h2` is for the top level headings within
a document. `h3` should be used for subheadings of `h2` headings.

.github/ISSUE_TEMPLATE/feature_request.md

@@ -4,24 +4,24 @@ about: Suggest a feature for this library
 labels: feature request
 ---
 
-### The Problem
+## The Problem
 
 <!--
 What problem is your feature trying to solve? What becomes easier or possible when this feature is implemented?
 -->
 
-### The Ideal Solution
+## The Ideal Solution
 
 <!--
 What is your ideal solution to the problem? What would you like this feature to do?
 -->
 
-### The Current Solution
+## The Current Solution
 
 <!--
 What is the current solution to the problem, if any?
 -->
 
-### Summary
+## Summary
 
 <!-- A short summary of your feature request. -->