Avoid YARD API annotation warnings
This PR amends the API reference documentation.
Before this change, these were the warnings:
Building YARD (yri) index for oauth2-1.4.4...
lib/oauth2/client.rb:17: [UnknownParam] @param tag has unknown parameter name: opts
lib/oauth2/client.rb:48: [UnknownParam] @param tag has unknown parameter name: the
lib/oauth2/client.rb:132: [UnknownParam] @param tag has unknown parameter name: access
lib/oauth2/client.rb:133: [UnknownParam] @param tag has unknown parameter name: class
lib/oauth2/mac_token.rb:12: [UnknownParam] @param tag has unknown parameter name: opts
lib/oauth2/mac_token.rb:79: [UnknownParam] @param tag has unknown parameter name: url. Did you mean `uri`?
lib/oauth2/access_token.rb:9: [UnknownParam] @param tag has unknown parameter name: the
lib/oauth2/access_token.rb:10: [UnknownParam] @param tag has unknown parameter name: a