docs : mark code as Bash (#3375)

docs/BLIS.md

@@ -48,7 +48,7 @@ make -j
 According to the BLIS documentation, we could set the following
 environment variables to modify the behavior of openmp:
 
-```
+```bash
 export GOMP_GPU_AFFINITY="0-19"
 export BLIS_NUM_THREADS=14
 ```