Fix the update-po task

[NickVolynkin]

What needs to be done

Reviewer: @patiencedaur

Definition of done

• make update-po updates localization files

Issue

docker run ... make update-po results in an error:

[ 50%] Built target update-pot
[100%] Regenerate localization files from templates
Traceback (most recent call last):
  File "/usr/local/bin/sphinx-intl", line 8, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.8/site-packages/click/core.py", line 1128, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/click/core.py", line 1053, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.8/site-packages/click/core.py", line 1656, in invoke
    super().invoke(ctx)
  File "/usr/local/lib/python3.8/site-packages/click/core.py", line 1395, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.8/site-packages/click/core.py", line 754, in invoke
    return __callback(*args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/click/decorators.py", line 26, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/sphinx_intl/commands.py", line 189, in main
    cfg = read_config(ctx.config, tag)
  File "/usr/local/lib/python3.8/site-packages/sphinx_intl/commands.py", line 45, in read_config
    execfile_(os.path.basename(path), namespace)
  File "/usr/local/lib/python3.8/site-packages/sphinx_intl/pycompat.py", line 57, in execfile_
    exec(code, _globals)
  File "conf.py", line 173, in <module>
    with open('../prolog.rst', 'r') as prolog:
FileNotFoundError: [Errno 2] No such file or directory: '../prolog.rst'
make[3]: *** [CMakeFiles/update-po.dir/build.make:77: CMakeFiles/update-po] Error 1
make[2]: *** [CMakeFiles/Makefile2:320: CMakeFiles/update-po.dir/all] Error 2
make[1]: *** [CMakeFiles/Makefile2:327: CMakeFiles/update-po.dir/rule] Error 2
make: *** [Makefile:215: update-po] Error 2