Stack underflow: specify 2 numbers are expected (#1395)

exercises/concept/stack-underflow/.docs/instructions.md

@@ -27,7 +27,7 @@ raise StackUnderflowError, "when dividing"
 
 ## 3. Write a dividing function
 
-Implement the `divide/1` function which takes a stack _(list of numbers)_ and:
+Implement the `divide/1` function which takes a stack _(a list of two numbers)_ and:
 
 - raises _stack underflow_ when the stack does not contain enough numbers
 - raises _division by zero_ when the divisor is 0 (note the stack of numbers is stored in the reverse order)

exercises/concept/stack-underflow/.meta/exemplar.ex

@@ -18,6 +18,6 @@ defmodule RPNCalculator.Exception do
   end
 
   def divide(stack) when length(stack) < 2, do: raise(StackUnderflowError, "when dividing")
-  def divide([divisor, _number | _]) when divisor == 0, do: raise(DivisionByZeroError)
-  def divide([divisor, number | _]), do: number / divisor
+  def divide([divisor, _number]) when divisor == 0, do: raise(DivisionByZeroError)
+  def divide([divisor, number]), do: number / divisor
 end