Merge pull request opencv#19586 from larryw3i:patch-2

* Update py_setup_in_ubuntu.markdown

Author: larryw3i

doc/py_tutorials/py_setup/py_setup_in_ubuntu/py_setup_in_ubuntu.markdown

@@ -22,10 +22,10 @@ Installing OpenCV-Python from Pre-built Binaries
 
 This method serves best when using just for programming and developing OpenCV applications.
 
-Install package [python-opencv](https://packages.ubuntu.com/trusty/python-opencv) with following command in terminal (as root user).
+Install package [python3-opencv](https://packages.ubuntu.com/focal/python3-opencv) with following command in terminal (as root user).
 
 ```
-$ sudo apt-get install python-opencv
+$ sudo apt-get install python3-opencv
 ```
 
 Open Python IDLE (or IPython) and type following codes in Python terminal.