fixes broken links in api documentation to LTI

fixes CNVS-13740

test plan
 - run rake doc:api
 - /doc/api/file.tools_intro.html should render
 - links to LTI resources should work

Change-Id: Ifb90b0f23ef84d3376fea445057ded2a256d820b
Reviewed-on: https://gerrit.instructure.com/36816
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>
QA-Review: Rob Orton <rob@instructure.com>