chore: update reference to copy() function in data-classes.md (#5437)

Author: SebastianAigner

docs/topics/data-classes.md

@@ -13,7 +13,7 @@ The compiler automatically derives the following members from all properties dec
 * `equals()`/`hashCode()` pair.
 * `toString()` of the form `"User(name=John, age=42)"`.
 * [`componentN()` functions](destructuring-declarations.md) corresponding to the properties in their order of declaration.
-* `copy()` function (see below).
+* [`copy()` function](#copying).
 
 To ensure consistency and meaningful behavior of the generated code, data classes have to fulfill the following requirements:
 
@@ -138,4 +138,4 @@ println("$name, $age years of age")
 ## Standard data classes
 
 The standard library provides the `Pair` and `Triple` classes. In most cases, though, named data classes are a better design choice
-because they make the code easier to read by providing meaningful names for the properties.
+because they make the code easier to read by providing meaningful names for the properties.