Update gnxi ptr (#15562)

#### Why I did it

Need new changes that were added to gnxi inside ptf docker

##### Work item tracking
- Microsoft ADO **(number only)**: 17747466

#### How I did it

Update commit number

#### How to verify it

Pipeline

dockers/docker-ptf/Dockerfile.j2

@@ -209,7 +209,7 @@ RUN mkdir -p /var/log/supervisor
 # Install Python-based GNMI client
 RUN git clone https://github.com/lguohan/gnxi.git \
     && cd gnxi \
-    && git checkout f2b11e4 \
+    && git checkout b85e4ab \
     && cd gnmi_cli_py \
     && pip install -r requirements.txt