Use preferred capitalization of "Slurm" (#41109)

https://slurm.schedmd.com/faq.html#acronym

lib/spack/docs/packaging_guide.rst

@@ -2337,7 +2337,7 @@ window while a batch job is running ``spack install`` on the same or
 overlapping dependencies without any process trying to re-do the work of
 another.
 
-For example, if you are using SLURM, you could launch an installation
+For example, if you are using Slurm, you could launch an installation
 of ``mpich`` using the following command:
 
 .. code-block:: console

lib/spack/spack/test/llnl/util/lock.py

@@ -18,7 +18,7 @@
     mpirun -n 7 spack test lock
 
 And it will test locking correctness among MPI processes.  Ideally, you
-want the MPI processes to span across multiple nodes, so, e.g., for SLURM
+want the MPI processes to span across multiple nodes, so, e.g., for Slurm
 you might do this::
 
     srun -N 7 -n 7 -m cyclic spack test lock