update LTI testing docs for access tokens
update LTI testing docs for access tokens

refs INTEROP-6870
flag=none

test plan:
* with an installation of the 1.3 test tool, the updated command
in these docs produces a valid LTI access token

Change-Id: I13ed4b6915fe9aeaab5069674d040a0ee8ba38fd
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/267244
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Alexandre Trindade <f185c528be8ad1468cdf5c6cccdc72d6de8c2461@instructure.com>
QA-Review: Xander Moffatt <27f70fe194da423604e6e0a63a3de19814736fa9@instructure.com>
Product-Review: Xander Moffatt <27f70fe194da423604e6e0a63a3de19814736fa9@instructure.com>

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