Update intro-risk.md

docs/intro-risk.md

@@ -56,7 +56,7 @@ Due to requests from regional teams, a refined calculation of the integral for p
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 align: center
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-The integral below the curve is calculated as the mean of lower and upper bound rectangles areas.
+The integral below the curve is calculated as the mean of lower and upper bound rectangles areas ([Riemann sum](https://www.math.net/riemann-sum)).
 ```
 
 ### Direct and indirect losses