Update the URL of script 'get-pip.py'

The URL of script 'get-pip.py' has changed.[1]

[1] https://ci.trustedfirmware.org/job/ci-dockerfiles-sanity-check/label=build-amd64/lastBuild/console

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: I9a958719895243e9845308dbc57777790b8f6cb3
diff --git a/bionic-amd64-tf-m-build/Dockerfile b/bionic-amd64-tf-m-build/Dockerfile
index 7afc57c..486241f 100644
--- a/bionic-amd64-tf-m-build/Dockerfile
+++ b/bionic-amd64-tf-m-build/Dockerfile
@@ -65,7 +65,7 @@
     apt update -q=2 ;\
     apt install -q=2 --yes --no-install-recommends git-lfs ;\
     # Install Python requirements
-    curl -s https://bootstrap.pypa.io/3.5/get-pip.py -o /tmp/get-pip.py ;\
+    curl -s https://bootstrap.pypa.io/pip/3.5/get-pip.py -o /tmp/get-pip.py ;\
     python2 /tmp/get-pip.py ;\
     pip2 install --no-cache-dir -r /opt/requirements_python2.txt ;\
     python3 /tmp/get-pip.py ;\