Readme contains unfechable wikidump

[JoWohlen]

While discribing step 4. of the docker process you mention:
The following example will process the 20200101 wiki dump snapshot for Somali language: make DATE=20200101 LANG=so DUMPDIR_BASE=/workspace/linked_volume/dump OUTDIR_BASE=/workspace/linked_volume/wiki PYTHONBIN=python all

--> this snapshot cant be fetched anymore (at least for me it failed).

• please add a note to try it with a newer dump if fetching it does not work
• please replace 20200101 wit for example 20220120 which does work