Update benchmarking/README.md

Co-authored-by: Adam Gleave <adam@gleave.me>

Author: timbauman

benchmarking/README.md

@@ -55,7 +55,7 @@ python experiments/commands.py \
   --output_dir=output | parallel -j 8
 ```
 
-(You may need to `brew install parallel` to get this to work on Mac)
+(You may need to `brew install parallel` to get this to work on Mac.)
 
 ### Running on Hofvarpnir