.azurepipelines: Fix cspell version to v5.20.0

.azurepipelines: Fix cspell version to v5.20.0

The v5.21.0 update introduced a number of new issues that might
impact stable tag integration so this fixes the version to the
previous release.

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
This value can be revisited in the future to update the