[ISSUE_TEMPLATE] Bigger headings

Don't be that shy. These bigger headings bring an underline with GitHub's stylesheets which gives the different sections better visual distinction.

.github/ISSUE_TEMPLATE.md

@@ -5,24 +5,24 @@
 
 * [ ] I've read and understood the [*CONTRIBUTING guidelines and have done my best effort to follow](https://github.com/CocoaPods/CocoaPods/blob/master/CONTRIBUTING.md).
 
-### Report
+# Report
 
-#### What did you do?
+## What did you do?
 
 > ℹ Please replace this with what you did.  
 > e.g. Run `pod install`
 
-#### What did you expected to happen?
+## What did you expected to happen?
 
 > ℹ Please replace this with what you expected to happen.  
 > e.g. Install all pod dependencies correctly.
 
-#### What happened instead?
+## What happened instead?
 
 > ℹ Please replace this with of what happened instead.  
 > e.g. Pod A is missing the subspec B for target C.
 
-### Podfile
+## Podfile
 
 > ℹ Please paste the contents of your Podfile below.    
 >  (e.g. via `cat Podfile | pbcopy`)