docs: fix syntax in cluster example (#2460)

### Description

Fix misleading syntax in documentation example. See
<https://www.biostars.org/p/9575301/>.

### QC
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* [ ] ~~The PR contains a test case for the changes or the changes are
already covered by an existing test case.~~ N.A.
* [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).

docs/executing/cluster.rst

@@ -120,8 +120,8 @@ Each of these can be part of a rule, e.g.:
       input: ...
       output: ...
       resources:
-          partition: <partition name>
-          runtime: <some number>
+          partition=<partition name>,
+          runtime=<some number>
 
 Please note: as ``--mem`` and ``--mem-per-cpu`` are mutually exclusive on SLURM clusters, there corresponding resource flags ``mem_mb`` and ``mem_mb_per_cpu`` are mutually exclusive, too.
 You can only reserve memory a compute node has to provide or the memory required per CPU (SLURM does not make any distintion between real CPU cores and those provided by hyperthreads).