Merge pull request #256 from ruby-go-gem/fix_doc

docs: Fix doc

Author: sue445

README.md

@@ -127,7 +127,7 @@ However, some of the methods listed below are not supported.
 
 ## Developing
 ### Build
-Run `bundle exec rake ruby:example:build`.
+Run `bundle exec rake build_all`.
 
 See `bundle exec rake -T` for more tasks.