Fix readme, in accordance to the updated tests

This is as described in exercism/problem-specifications#1395
The current README will really confuse non math-savvy students, as it asked to find `a*b*c`, while the tests actually check for the triplets.
Change it to asks for the triplet explicitly. Give the answer for N = 1000 as it's also spoiled in the test file ;)

Author: fudanchii

exercises/pythagorean-triplet/README.md

@@ -19,9 +19,9 @@ For example,
 3**2 + 4**2 = 9 + 16 = 25 = 5**2.
 ```
 
-There exists exactly one Pythagorean triplet for which a + b + c = 1000.
+Given a number `N`, find all triplets `[a, b, c]` that satisfy `a + b + c = N`.
 
-Find the product a * b * c.
+For example, for `N = 1000`, there is only 1 triplet that satisfies `a + b + c = 1000`, that is: `[200, 375, 425]`
 
 ## Rust Installation