doc: improve issue template

The previous template used an ordered list, but the formatting always
breaks when users paste quoted snippets of code or command outputs.
It is also harder to visually parse because items in ordered lists
are only indented but not highlighted in any way.

Change-Id: I73c89e9f0465aef41093f5c54d11bb0d12ff8c8d
Reviewed-on: https://go-review.googlesource.com/27252Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

.github/ISSUE_TEMPLATE

 Please answer these questions before submitting your issue. Thanks!
 
-1. What version of Go are you using (`go version`)?
+### What version of Go are you using (`go version`)?
 
 
-2. What operating system and processor architecture are you using (`go env`)?
+### What operating system and processor architecture are you using (`go env`)?
 
 
-3. What did you do?
+### What did you do?
 If possible, provide a recipe for reproducing the error.
 A complete runnable program is good.
 A link on play.golang.org is best.
 
 
-4. What did you expect to see?
+### What did you expect to see?
 
 
-5. What did you see instead?
+### What did you see instead?