Fix rails "onbuid" typo

Closes docker-library/rails#23

rails/README.md

@@ -49,7 +49,7 @@ browser.
 
 ### Generate a `Gemfile.lock`
 
-The `onbuid` tag expects a `Gemfile.lock` in your app directory. This `docker
+The `onbuild` tag expects a `Gemfile.lock` in your app directory. This `docker
 run` will help you generate one. Run it in the root of your app, next to the
 `Gemfile`:
 

rails/content.md

@@ -38,7 +38,7 @@ browser.
 
 ### Generate a `Gemfile.lock`
 
-The `onbuid` tag expects a `Gemfile.lock` in your app directory. This `docker
+The `onbuild` tag expects a `Gemfile.lock` in your app directory. This `docker
 run` will help you generate one. Run it in the root of your app, next to the
 `Gemfile`: