Update README.md

Updated the relative links in ## Contents to point to the correct documents.

doc/api/README.md

@@ -69,18 +69,18 @@ When listing resources you can pass the following parameters:
 
 ## Contents
 
-+ [Users](api/users.md)
-+ [Session](api/session.md)
-+ [Projects](api/projects.md)
-+ [Project Snippets](api/project_snippets.md)
-+ [Repositories](api/repositories.md)
-+ [Issues](api/issues.md)
-+ [Milestones](api/milestones.md)
-+ [Notes](api/notes.md)
-+ [Deploy Keys](api/deploy_keys.md)
-+ [System Hooks](api/system_hooks.md)
-+ [Groups](api/groups.md)
-+ [User Teams](api/user_teams.md)
++ [Users](users.md)
++ [Session](session.md)
++ [Projects](projects.md)
++ [Project Snippets](project_snippets.md)
++ [Repositories](repositories.md)
++ [Issues](issues.md)
++ [Milestones](milestones.md)
++ [Notes](notes.md)
++ [Deploy Keys](deploy_keys.md)
++ [System Hooks](system_hooks.md)
++ [Groups](groups.md)
++ [User Teams](user_teams.md)
 
 ## Clients