[bootlin/training-materials] 9773f8: yocto: slides: improve PREFERRED_VERSION example

[Luca Ceresoli]

  Branch: refs/heads/master
  Home:   https://github.com/bootlin/training-materials
  Commit: 9773f8efd6b04a71bed8efa58f2e77480377e8aa
      https://github.com/bootlin/training-materials/commit/9773f8efd6b04a71bed8efa58f2e77480377e8aa
  Author: Luca Ceresoli <luca.ceresoli at bootlin.com>
  Date:   2022-07-18 (Mon, 18 Jul 2022)

  Changed paths:
    M slides/yocto-advanced/yocto-advanced.tex

  Log Message:
  -----------
  yocto: slides: improve PREFERRED_VERSION example

Python 2 is really EOL now and it's not in any layer we use in the
examples. Now there is only Python 3, the recipe is called "python3" but it
implements one version only. For a more realistic example, switch to a
package that currently has at least two versions. Unfortunately oe-core has
no such recipe, thus pick one from meta-openembedded, which is nginx having
the "stable" and "current" versions.

Signed-off-by: Luca Ceresoli <luca.ceresoli at bootlin.com>