Remove "--break-system-packages" flag from mypy install in README.md

- Rationale: On Ubuntu 22.04 is an older version of the pip3 and python
version which doesn't have this flag.
- Also removed sudo before "pip3 install -U mypy==1.9" in README.md

Signed-off-by: Michael Orlov <michael.orlov@apex.ai>

rosbag2_py/README.md

@@ -18,7 +18,7 @@ python3 -c 'import rosbag2_py'
 sudo apt update && sudo apt install mypy
 
 # Older Ubuntu
-# sudo pip3 install -U --break-system-packages mypy==1.9
+# pip3 install -U mypy==1.9
 
 cd <rosbag2 git repo>
 stubgen -p rosbag2_py -o rosbag2_py