Update instructions.append.md (exercism#3947)

Instruction does not match the unit test. The exception message tested in the unit test is more precise. Hence, I've changed the instruction.

Author: jmh-git

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

@@ -10,7 +10,7 @@ To raise a `ValueError` with a message, write the message as an argument to the
 
 ```python
 # span of numbers is longer than number series
-raise ValueError("span must be smaller than string length")
+raise ValueError("span must not exceed string length")
 
 # span of number is negative
 raise ValueError("span must not be negative")