Fix SLURM_CLUSTER_NAME variable name in slurm.conf man page

The environment variable name SLURM_JOB_CLUSTER_NAME should be
SLURM_CLUSTER_NAME. This is also available in Prolog and Epilog, so
remove note about it only being available in PrologSlurmctld and
EpilogSlurmctld.

doc/man/man5/slurm.conf.5

@@ -3791,14 +3791,13 @@ SLURM_ARRAY_JOB_ID with SLURM_ARRAY_TASK_ID
 (e.g. "scontrol update ${SLURM_ARRAY_JOB_ID}_{$SLURM_ARRAY_TASK_ID} ...");
 Available in \fBPrologSlurmctld\fR and \fBEpilogSlurmctld\fR only.
 .TP
+\fBSLURM_CLUSTER_NAME\fR
+Name of the cluster executing the job.
+.TP
 \fBSLURM_JOB_ACCOUNT\fR
 Account name used for the job.
 Available in \fBPrologSlurmctld\fR and \fBEpilogSlurmctld\fR only.
 .TP
-\fBSLURM_JOB_CLUSTER_NAME\fR
-Name of the cluster executing for the job.
-Available in \fBPrologSlurmctld\fR and \fBEpilogSlurmctld\fR only.
-.TP
 \fBSLURM_JOB_CONSTRAINTS\fR
 Features required to run the job.
 Available in \fBPrologSlurmctld\fR and \fBEpilogSlurmctld\fR only.