Android: Use XR instead of VR for xr_extensions README.md

Test: N/A

Change-Id: Ie1db3c78e987edce7750b1b4257106c1f2a7bcbd
Signed-off-by: utzcoz <utzcoz@gmail.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6293945
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Sam Maier <smaier@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1423899}
NOKEYCHECK=True
GitOrigin-RevId: ed3cd9a5bfb052d8086d0f9e1844a88f1b4e2b6c