Fix typo

Author: cllns

spec/support/os.rb

@@ -13,7 +13,7 @@ def with_operating_system(os, &blk)
                  when /darwin/ then :macos
                  when /win32|mingw|bccwin|cygwin/ then :windows
                  else
-                   raise "unkwnown OS: `#{host_os}'"
+                   raise "unknown OS: `#{host_os}'"
                  end
 
         blk.call if result == os