shorten file extension in gemspec

Author: dchelimsky

rspec.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |s|
   s.files           += ["License.txt"]
   s.test_files       = `git ls-files -- {spec,features}/*`.split("\n")
   s.executables      = `git ls-files -- bin/*`.split("\n").map{ |f| File.basename(f) }
-  s.extra_rdoc_files = [ "README.markdown" ]
+  s.extra_rdoc_files = [ "README.md" ]
   s.rdoc_options     = ["--charset=UTF-8"]
   s.require_path     = "lib"