💚 Fix Rubygems.org action

the name of the readme file is incorrect

arctic_admin.gemspec

@@ -9,7 +9,7 @@ Gem::Specification.new do |s|
   s.summary     = "Arctic Admin theme for ActiveAdmin"
   s.description = "A responsive theme for Active Admin"
   s.authors     = ["Clément Prod'homme"]
-  s.files       = Dir["{app,lib}/**/*"] + ["Readme.md", 'LICENCE.txt']
+  s.files       = Dir["{app,lib}/**/*"] + ["README.md", 'LICENCE.txt']
   s.homepage    = 'https://github.com/cprodhomme/arctic_admin'
   s.license     = 'MIT'
   s.require_paths = ["lib"]