Update rspec requirement from ~> 3.12.0 to ~> 3.13.0

Updates the requirements on [rspec](https://github.com/rspec/rspec-metagem) to permit the latest version.
- [Commits](rspec/rspec-metagem@v3.12.0...v3.13.0)

---
updated-dependencies:
- dependency-name: rspec
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

rack-ecg.gemspec

@@ -39,7 +39,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency("rackup", "~> 2.1.0")
   spec.add_development_dependency("rake", "~> 13.0")
   spec.add_development_dependency("redcarpet", "~> 3.6.0")
-  spec.add_development_dependency("rspec", "~> 3.12.0")
+  spec.add_development_dependency("rspec", "~> 3.13.0")
   spec.add_development_dependency("rubocop-shopify", "~> 2.14")
   spec.add_development_dependency("yard", "~> 0.9.34")
 end