Minor changes

Hey

I made small change in the guides, I removed a duplicated `require` in the `class HelloEndpoint`. 

The guides look great, they helped me to understand better the integration flow. Thanks!

Although the source code is available on GitHub, I was copying and pasting the code snippets and creating the files, since all file names are capitalized in the guides, I had to manually convert them to downcase. Maybe if they were in downcase, it would be easier. I committed a typo rewriting `fulfillment_endpoint`. :panda_face: