Made new replacement script executable

Author: mvhulten

README.md

@@ -248,7 +248,7 @@ Only for BGC mode!
 The following script updates landcover using GLC2000 and soils using SOILGRIDS.
 
 ```
-replace_surfdata.py
+./replace_surfdata.py
 ```
 
 Additionally, this script checks CLM gridcells to make sure the

mksurfdata/replace_surfdata.py

@@ -1,3 +1,4 @@
+#!/usr/bin/env python3
 import xarray as xr
 import numpy as np
 # import sys