Update formulas.py

Fix syntax for power operator to **

app/api/formulas.py

@@ -20,7 +20,7 @@ algos = {
         'help': _('Enhanced Normalized Difference Vegetation Index is like NDVI, but uses Blue and Green bands instead of only Red to isolate plant health.')
     },
     'vNDVI':{
-        'expr': '0.5268*((R ^ -0.1294) * (G ^ 0.3389) * (B ^ -0.3118))',
+        'expr': '0.5268*((R ** -0.1294) * (G ** 0.3389) * (B ** -0.3118))',
         'help': _('Visible NDVI is an un-normalized index for RGB sensors using constants derived from citrus, grape, and sugarcane crop data.')
     },
     'VARI': {