Update example of #find method in introduction.md (#1804)

Removes the potential for conveying that Enumerable#find returns the index of the found element rather than the element itself.

Forum discussion: https://forum.exercism.org/t/ambiguity-in-the-advanced-enumeration-learning-curriculum-boutique-inventory-learning-exercise/19966/8

Author: bradymcnellis

exercises/concept/boutique-inventory/.docs/introduction.md

@@ -15,7 +15,7 @@ fibonacci.select { |number| number.odd? }   #=> [1, 1, 3, 5, 13]
 fibonacci.all?   { |number| number < 20 }   #=> true
 fibonacci.map    { |number| number * 2  }   #=> [0, 2, 2, 4, 6, 10, 16, 26]
 fibonacci.select { |number| number >= 5}    #=> [5, 8, 13]
-fibonacci.find   { |number| number >= 5}    #=> 5
+fibonacci.find   { |number| number >= 6}    #=> 8
 
 # Some methods work with or without a block
 fibonacci.sum  #=> 33