Undid changes in the README

Changed the isosceles description back to the original.

exercises/triangle/README.md

 
 An _equilateral_ triangle has all three sides the same length.
 
-An _isosceles_ triangle has at least two sides the same length.
+An _isosceles_ triangle has at least two sides the same length. (It is sometimes
+specified as having exactly two sides the same length, but for the purposes of
+this exercise we'll say at least two.)
 
 A _scalene_ triangle has all sides of different lengths.