Clarify usage of option "-call"

[fhg-isi]

The option "-call" that is used for the tutorials might be misleading, as "call" could be interpretet as single word like in "call a function" and when searching for it in the snakemake documentaiton, it cannot be found. Also see

PyPSA/PyPSA#1060

Current version of snakemake does not seem to require the --cores (-c) option any more and uses all cores by default:

snakemake/snakemake#3167

=> I would prefer one of the following variants to avoid confusion by "black magic" command snakemake -call :

snakemake
snakemake --cores
snakemake --coresall
snakamake -c

[lkstrp]

While it still can be used, -call is indeed optional and not documented anymore. Thank you @fhg-isi !