fix: GroupedLayerControl docstring has improper RST formatting causing docs build failure

[Kakarot35]

Bug description

The docstring in folium/plugins/groupedlayercontrol.py has improper
RST indentation that causes Sphinx to fail during the docs build with:

ERROR: Unexpected indentation at line 6
WARNING: Block quote ends without a blank line; unexpected unindent at line 8

To reproduce

Run sphinx-build on the docs folder and observe the error in
folium.plugins.groupedlayercontrol.GroupedLayerControl

Expected behavior

Docs should build successfully without any RST formatting errors.

Actual behavior

Docs build fails due to improper indentation in the GroupedLayerControl
docstring example block.

Fix

Update the docstring to use proper RST formatting with correct
indentation and :: for the code block example.