Docserver: Rename copresenceSocket -> copresenceEndpoints

The API was renamed a while ago, but the doc template was never
updated.

BUG=None
TBR=kalman@chromium.org
TBR=rkc@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/829333005

Cr-Commit-Position: refs/heads/master@{#310184}

chrome/common/extensions/docs/templates/public/apps/copresenceEndpoints.html

+{{+partials.standard_apps_api api:apis.apps.copresence_endpoints/}}

chrome/common/extensions/docs/templates/public/apps/copresenceSocket.html

-{{+partials.standard_apps_api api:apis.apps.copresence_socket/}}