Fix docstring for Euler problem 11, solution 1 (#3228)

Docstring must have been leftover from some FizzBuzz method.
Now it describes the actual code.

project_euler/problem_11/sol1.py

@@ -68,7 +68,8 @@ def largest_product(grid):
 
 
 def solution():
-    """Returns the sum of all the multiples of 3 or 5 below n.
+    """Returns the greatest product of four adjacent numbers (horizontally,
+    vertically, or diagonally).
 
     >>> solution()
     70600674