Fix typo (AMReX-Codes#1292)

## Summary
Fix a typo in AMReX_extrapolater_3D_K.H

## Additional background

## Checklist

The proposed changes:
- [x] fix a bug or incorrect behavior in AMReX
- [ ] add new capabilities to AMReX
- [ ] changes answers in the test suite to more than roundoff level
- [ ] are likely to significantly affect the results of downstream AMReX users
- [ ] are described in the proposed changes to the AMReX documentation, if appropriate

Src/Amr/AMReX_extrapolater_3D_K.H

@@ -753,7 +753,7 @@ amrex_first_order_extrap_gpu(int i, int j, int k, int n,
             data(i,j,k,n) = (  mask(i-1,j+1,k) * data(i-1,j+1,k,n) 
                              + mask(i-1,j,k+1) * data(i-1,j,k+1,n) 
                              + mask(i,j+1,k+1) * data(i,j+1,k+1,n) )
-                            / ( mask(i-1,j+1,k) + mask(i+1,j,k+1) + mask(i,j+1,k+1) );
+                            / ( mask(i-1,j+1,k) + mask(i-1,j,k+1) + mask(i,j+1,k+1) );
          } else {
             data(i,j,k,n) = data(i-1,j+1,k+1,n);
          }