Fix the url to access sample container in BUILD-JAVA

Fix simple typo in the url for accessing sample container with Jetty plugin in BUILD-JAVA instruction text.

Before:

• http://localhost:8080/gadgets/files/samplecontainer/samplecontainer.html

After:
+ http://localhost:8080/samplecontainer/samplecontainer.html

The shindig-server generate war file with samplecontainer in the root web directory.