Update README.MD

current bug in snakemake 7 causes conda environments to build simultaneously within cluster jobs snakemake/snakemake#1463

README.MD

@@ -40,7 +40,7 @@ $ conda create \
     --channel bioconda \
     --channel conda-forge \
     --prefix ./conda_env \
-    python=3 snakemake mamba lockfile
+    python=3 snakemake=6 mamba lockfile
 
 # activate the base conda environment
 $ conda activate ./conda_env