The flag --hugo-section-files does not result in file rename

[g-pavlov]

What happened:
Applying --hugo-section-files=README.md does not lead to renaming an existing README.md to _index.md as expected.

What you expected to happen:
the README.md should have been renamed to _index.md

How to reproduce it (as minimally and precisely as possible):
manifest:

nodesSelector:
  path: https://github.com/gardener/gardener/tree/master/docs    

use the --hugo and --hugo-section-files=README.md flags

The README.md repo file should become _index.md

How to categorize this issue?
/kind bug
/priority critical