[MINOR] Fixed tickbox syntax in PR template (#131)

Signed-off-by: Adrian Sutton <adrian.sutton@consensys.net>

.github/pull_request_template.md

@@ -20,10 +20,10 @@ Fixes #
 
 <!--- What types of changes does your code introduce? Put an `x` in all the boxes that apply: -->
 
-- [] Docs change / refactoring / dependency upgrade
-- [] Bug fix (non-breaking change which fixes an issue)
-- [] New feature (non-breaking change which adds functionality)
-- [] Breaking change (fix or feature that would cause existing functionality to change)
+- [ ] Docs change / refactoring / dependency upgrade
+- [ ] Bug fix (non-breaking change which fixes an issue)
+- [ ] New feature (non-breaking change which adds functionality)
+- [ ] Breaking change (fix or feature that would cause existing functionality to change)
 
 ## Release note
 <!--  Write your release note: