Docs: Tutorial General Use - Tool Wrappers, Example reference to outdated version

While following the tutorial General Use, In [Tool Wrappers](https://snakemake.readthedocs.io/en/stable/tutorial/additional_features.html#tool-wrappers) section, I think for the rule, rule bwa_mem example code is out of date, the following line should be updated: 
"0.15.3/bio/bwa/mem"

