TF-M Build: Set docutils version to 0.16

The default version of docutils in docker is 0.17.1.
TF-M docs could not generate expected bullet lists with this version.
Set docutils version to 0.16 in dockerfile.

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I0140cd4c4e799923a42358fc1f0d5ed74a59d83d
1 file changed