fix variable name to nvidia_gpu

Author: nicolasboisseau

docs/further.md

@@ -107,9 +107,9 @@ The following environment variables are available within any Google batch run:
 
 The Google Batch executor uses the same designation for GPUs as core Snakemake. However, you should 
 [keep compatibility of machine type](https://cloud.google.com/compute/docs/gpus) with the GPU
-that you selected in mind. For example, if you select `gpu_nvidia=1` you will need an n1-* family machine type.
+that you selected in mind. For example, if you select `nvidia_gpu=1` you will need an n1-* family machine type.
 
-On a n1-* family machine type, gpu_nvidia=1 will trigger a "nvidia-tesla-t4" gpu by default.
+On a n1-* family machine type, nvidia_gpu=1 will trigger a "nvidia-tesla-t4" gpu by default.
 
 It's possible to change the gpu type directly using a machine-compatible label:
 e.g. nvidia_gpu='nvidia-tesla-v100'