Fix documentation on variable substitution
Fix documentation on variable substitution

Test plan:
n/a

Change-Id: Ib9133f16af8999c5cf832483d2d49b0134b121f7
Reviewed-on: https://gerrit.instructure.com/83790
Tested-by: Jenkins
Reviewed-by: Nathan Mills <10b0771d20c18ad0d98c1d000665e443052b40b0@instructure.com>
Product-Review: Andrew Butterfield <c9c7a32d9a1e9779cfad12bed963960e7af36eac@instructure.com>
QA-Review: Andrew Butterfield <c9c7a32d9a1e9779cfad12bed963960e7af36eac@instructure.com>

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