fix the debian instructions

* "basé" is anglicism (on based) for the french word "fondé"
* `apt-get` comes from the old apt tools. is not aptitude. if you have
  aptitude installed on your system, then you know why you installed
  it and you probably don't need those instructions. so the recommended way
  for newbies to install a package is `apt` nowadays.
* guile 3 is only available on testing/unstable versions of debian.
  if you really want guile 3 on it, i really recommand to use
  debootstrap and chroot.
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