update debian package name for sphinx

doc/building.rst

@@ -27,7 +27,7 @@ Building with CMake on Ubuntu or Debian
    # enable additional features
    sudo apt-get install liburcu-dev # for multithreading
    sudo apt-get install check # for unit tests
-   sudo apt-get install sphinx graphviz # for documentation generation
+   sudo apt-get install python-sphinx graphviz # for documentation generation
    sudo apt-get install python-sphinx-rtd-theme # documentation style
 
    cd open62541