typo in jsx README

Change-Id: Ieb380a5c657d2fd13437afc45f281b058febd739
Reviewed-on: https://gerrit.instructure.com/43847
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Ryan Florence <ryanf@instructure.com>
Product-Review: Ryan Florence <ryanf@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>