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
diff --git a/bionic-amd64-tf-m-build/requirements_python3.txt b/bionic-amd64-tf-m-build/requirements_python3.txt
index 4f43ff1..0ced815 100644
--- a/bionic-amd64-tf-m-build/requirements_python3.txt
+++ b/bionic-amd64-tf-m-build/requirements_python3.txt
@@ -1,5 +1,6 @@
 cbor==1.0.0
 cryptography==2.7
+docutils==0.16
 Jinja2==2.10
 MarkupSafe==1.1.0
 PyYAML==3.12