docs : fix typo GOMP_CPU_AFFINITY (#3597)

Author: maekawatoshiki

docs/BLIS.md

@@ -49,7 +49,7 @@ 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 GOMP_CPU_AFFINITY="0-19"
 export BLIS_NUM_THREADS=14
 ```