math.statistics-docs: make cum-product example less ambiguous

In the previous version of the example it was not clear whether the result
always starts with 1 (as is the case with cum-product1) or with the first
item of the input sequence.

basis/math/statistics/statistics-docs.factor

@@ -173,8 +173,8 @@ HELP: cum-product
 { $description "Returns the cumulative product of " { $snippet "seq" } "." }
 { $examples
     { $example "USING: math.statistics prettyprint ;"
-               "{ 1 2 3 4 } cum-product ."
+               "{ 2 3 4 } cum-product ."
-               "{ 1 2 6 24 }"
+               "{ 2 6 24 }"
     }
 } ;