docs: fix explanation on how to use all cores in tutorial (#2733)

### Description

The usage of the CLI parameter --cores changed. I adapted the
description in tutorial to that change.

### QC
<!-- Make sure that you can tick the boxes below. -->

* [ ] ~The PR contains a test case for the changes or the changes are
already covered by an existing test case.~
* [x] The documentation (`docs/`) is updated to reflect the changes or
this is not necessary (e.g. if the change does neither modify the
language nor the behavior or functionalities of Snakemake).

Co-authored-by: Jan Hünnemeyer <506283-horstage@users.noreply.gitlab.com>

docs/tutorial/advanced.rst

@@ -76,7 +76,7 @@ would execute the workflow with 10 cores.
 Since the rule ``bwa_map`` needs 8 threads, only one job of the rule can run at a time, and the Snakemake scheduler will try to saturate the remaining cores with other jobs like, e.g., ``samtools_sort``.
 The threads directive in a rule is interpreted as a maximum: when **less cores than threads** are provided, the number of threads a rule uses will be **reduced to the number of given cores**.
 
-If ``--cores`` is given without a number, all available cores are used.
+If ``--cores all`` is given, all available cores are used.
 
 Exercise
 ........