docs(generated): fix OpenAPI spec link (#312)

Without a protocol, this link is interpreted as a relative URL, so if
the file is viewed at
https://github.com/octokit/plugin-rest-endpoint-methods.js/blob/master/src/generated/README.md
the link resolves to
https://github.com/octokit/plugin-rest-endpoint-methods.js/blob/master/src/generated/github.com/github/rest-api-description
which does not exist.  This commit adds the https:// protocol so the
link always resolve to https://github.com/github/rest-api-description

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>

Author: kevinoid

src/generated/README.md

@@ -1,3 +1,3 @@
 # DO NOT EDIT FILES IN THIS DIRECTORY
 
-All files are generated automatically and will be overwritten next time [GitHub's OpenAPI spec](github.com/github/rest-api-description/) has a new release. If you find a problem, please file an issue.
+All files are generated automatically and will be overwritten next time [GitHub's OpenAPI spec](https://github.com/github/rest-api-description/) has a new release. If you find a problem, please file an issue.