Placeholder URL for Sentry API input field is incorrect

The following discussion from !23770 (merged) should be addressed:

• @ClemMakesApps started a discussion:

  Also, I noticed that the placeholder for the sentry API URL is incorrect.

  As discussed in https://gitlab.slack.com/archives/C1ZCCRWBC/p1547246320226500, we should omit the /issues/ part of the path

  cc @tristan.read