Adapt to new pluralize routes

[pierreburgy]

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Improvement.

• What is the current behavior? (You can also link to an open issue here)

The documentation is adapted to singular routes.

• What is the new behavior (if this is a feature change)?

Since now, Strapi API endpoints are pluralized. So, the SDK documentation needs to be updated.

• Other information:

Related to strapi/strapi#1725.

[lucaperret]

Thanks @pierreburgy for this update !