fix typo

Author: jswhit

.github/workflows/build-wheels.yml

@@ -35,7 +35,7 @@ jobs:
         exclude:
           - os: macos-latest  
             platform: x32
-          - ML_MB_VER: 1
+          - MB_ML_VER: 1
             python-version: ["3.7","3.8","3.9"]
     env:
       REPO_DIR: netcdf4-python