Fix typo

README.md

@@ -57,6 +57,6 @@ Install NodeJS + npm
 
 # About the attractiveness calculation
 
-For each region and each factor, the index of attractiveness is _idx<sub>f</sub> = &sum; ds<sub>i</sub> / N<sub>ds</sub>_ , where _ds<sub>i</sub>_ is a normalized value for dataset _i_ and _N<sub>ds</sub> is a number of datasets in the factor.
+For each region and each factor, the index of attractiveness is _idx<sub>f</sub> = &sum; ds<sub>i</sub> / N<sub>ds</sub>_ , where _ds<sub>i</sub>_ is a normalized value for dataset _i_ and _N<sub>ds</sub>_ is a number of datasets in the factor.
 
 For each region the aggregated index of attractiveness is then _idx<sub>a</sub> = (&sum; (&sum; ds<sub>i</sub> * w<sub>f</sub>)) / (&sum; N<sub>ds</sub> * w<sub>f</sub>)_, where _w<sub>f</sub>_ is a weight of the factor.