fix an readme empty link

[oshliaer]

Fix the empty link to Smartsheet API docs

FILE: ./README.md [✓] https://travis-ci.org/googleworkspace/apps-script-oauth2 [✓] https://travis-ci.org/googleworkspace/apps-script-oauth2.svg?branch=master [✓] https://developers.google.com/apps-script/reference/script/state-token-builder [✓] https://developers.google.com/apps-script/reference/calendar/ [✓] https://developers.google.com/apps-script/advanced/admin-sdk-directory [✓] https://developers.google.com/apps-script/concepts/manifests#editing_a_manifest [✓] https://developers.google.com/apps-script/concepts/scopes#setting_explicit_scopes [✓] https://developers.google.com/apps-script/reference/script/script-app#getoauthtoken [✓] samples/NoLibrary [✓] dist [✓] samples/Add-on/Callback.html#L47 [✓] https://developers.google.com/apps-script/reference/properties/properties-service [✓] https://developers.google.com/gmail/add-ons/ [✓] http://googleworkspace.github.io/apps-script-oauth2/Service_.html [✓] samples/FitBit.gs [✓] https://smartsheet-platform.github.io/api-docs/#request-an-access-token [✓] samples/Smartsheet.gs [✓] https://tools.ietf.org/html/draft-ietf-oauth-jwt-bearer-12 [✓] https://developers.google.com/identity/protocols/OAuth2ServiceAccount#delegatingauthority [✓] samples/GoogleServiceAccount.gs [✓] samples/TwitterAppOnly.gs 21 links checked.

[erickoledadevrel]

Thanks for the fix!