docs/manual: don't mention eglibc, mention musl

docs/manual: don't mention eglibc, mention musl

Following the removal of eglibc, it no longer makes sense to talk about
it in the Buildroot manual as one of the supported C libraries. However,
it does make sense to indicate that we support musl.

[Peter: remove extra 'the' as suggested by Yann]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>