Update slurmmanager.jl

Author: DilumAluthge

src/slurmmanager.jl

@@ -21,10 +21,10 @@ end
 
 function get_slurm_ntasks_int()
   if haskey(ENV, "SLURM_NTASKS")
-    ntasks_str = ENV["SLURM_JOB_ID"]
+    ntasks_str = ENV["SLURM_NTASKS"]
   else
     msg = "SlurmManager must be constructed inside a Slurm allocation environment." *
-          "SLURM_JOB_ID or SLURM_JOBID must be defined."
+          "SLURM_NTASKS must be defined."
     error(msg)
   end
   ntasks_int = parse(Int, ntasks_str)::Int