docs/using.rst: remove note on mandatory .raucb suffix
docs/using.rst: remove note on mandatory .raucb suffix

This is no longer valid since commit 9660b57c1959f62b1c584e483e29d94fe2486b47
('src/main: do not reject local bundles that do not have a .raucb extension')

Signed-off-by: Enrico Joerns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

jluebbe/rauc,jluebbe/rauc,rauc/rauc,ejoerns/rauc,ejoerns/rauc,ejoerns/rauc,rauc/rauc,rauc/rauc,jluebbe/rauc