Add `padding: false` to Ruby base64 example

What does this MR do?

This MR corrects a Ruby example of a "URL-safe base64-encoded string of the SHA256 hash" in the "GitLab as an OAuth2 provider" documentation.

While implementing an OAuth application, I found that the base64-encoded SHA256 hash from the example Ruby code wasn't accepted by GitLab. I looked at the doorkeeper code and found that padding should be set to false.

https://github.com/doorkeeper-gem/doorkeeper/blob/main/lib/doorkeeper/models/access_grant_mixin.rb#L92

Screenshots or Screencasts (strongly suggested)

Not applicable, as this is a documentation change

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have self-reviewed this MR per code review guidelines.
• I have followed the style guides.

Availability and Testing

I reviewed these tasks and determined they were not applicable, as this is a documentation MR.