hello-world: don't call String hello a function (#1148)

Although function values and non-function values (and the variables
bound to them) have a lot in common in Haskell, we shouldn't call all of
them "functions".

The discussion at
https://forum.exercism.org/t/is-a-string-value-in-haskell-a-function/4034
resulted in this conclusion

exercism/problem-specifications#1850
exercism/problem-specifications#1926

Author: das-g

exercises/practice/hello-world/.docs/instructions.append.md

@@ -1,6 +1,6 @@
 # Hints
 
-To complete this exercise, you need to implement the `hello` function.
+To complete this exercise, you need to define the constant `hello`.
 
 You will find the type signature for `hello` already in place,
-but it is up to you to define the function.
+but it is up to you to define its value.

exercises/practice/hello-world/.docs/instructions.md

@@ -8,7 +8,7 @@ or environment.
 
 The objectives are simple:
 
-- Write a function that returns the string "Hello, World!".
+- Modify the provided code so that it produces the string "Hello, World!".
 - Run the test suite and make sure that it succeeds.
 - Submit your solution and check it at the website.