bionic-amd64-tf-m-build: Update to m2r2 0.3.2

"m2r2" package (as required for TF-M docs generation) has got a dependency
on "mistune" package, which in the meantime seems to have had a non
backwards compatible update. Version 0.3.2 of "m2r2" has the only change
comparing to the previously used 0.3.1 - pinning the right version of
"mistune" as a dependency:

https://github.com/CrossNox/m2r2/compare/v0.3.1...v0.3.2

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I1ddceb21ec881a1722e75488d07191b4856f741e
diff --git a/bionic-amd64-tf-m-build/requirements_python3.txt b/bionic-amd64-tf-m-build/requirements_python3.txt
index 14304f6..8fc2b0f 100755
--- a/bionic-amd64-tf-m-build/requirements_python3.txt
+++ b/bionic-amd64-tf-m-build/requirements_python3.txt
@@ -6,7 +6,7 @@
 PyYAML==5.4.1
 pycryptodome==3.6.6
 pyasn1==0.1.9
-m2r2==0.3.1
+m2r2==0.3.2
 Sphinx==4.2.0
 sphinx-rtd-theme==1.0.0
 sphinx-tabs==3.2.0