Synchronize example code with tests

Author: MatthijsBlom

exercises/practice/largest-series-product/.docs/instructions.append.md

@@ -12,8 +12,8 @@ To raise a `ValueError` with a message, write the message as an argument to the
 # span of numbers is longer than number series
 raise ValueError("span must be smaller than string length")
 
-# span of number is zero or negative
-raise ValueError("span must be greater than zero")
+# span of number is negative
+raise ValueError("span must not be negative")
 
 # series includes non-number input
 raise ValueError("digits input must only contain digits")