doc: fix references to distro documentation

Commit 37c5195d ("doc: Move distro boot doc to rST") renamed
doc/README.distro to doc/develop/distro.rst.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Soeren Moch <smoch@web.de>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>