chore: Tweak punctuation in variables6.rs hint
While the meaning is still obvious as is, it makes a little more sense
to use a colon here =)
@@ -79,7 +79,7 @@ path = "exercises/variables/variables6.rs"
mode = "compile"
hint = """
We know about variables and mutability, but there is another important type of
-variable available; constants.
+variable available: constants.
Constants are always immutable and they are declared with keyword 'const' rather
than keyword 'let'.
Constants types must also always be annotated.
