docs: correct the design type in the PR template (#6813)

The template offers `desig(scope)`, but the title check in
pull-request-label.yml accepts only feat|fix|chore|ref|clean|design|docs|
test|perf. A contributor who follows the template for a design proposal
therefore lands a red CI on the first push, which is what happened on
#6812.

Every other type in the template matches the workflow; this is the one
that does not.

Signed-off-by: sanchpet <petrov@sanch.pet>
Co-authored-by: Alexander Chernov <alexander@chernov.it>

.github/pull_request_template.md

@@ -22,7 +22,7 @@ ref(scope): refactor code
 clean(scope): provider cleanup
 test(scope): add tests
 perf(scope): improve performance
-desig(scope): improve design
+design(scope): improve design
 ```
 
 Where `scope` is _optionally_ one of: