chore: adjust tour practice headers

Author: sarahhaggarty

docs/topics/tour/kotlin-tour-intermediate-classes-interfaces.md

@@ -351,7 +351,7 @@ you don't have to add new lines of code for every inherited member function.
 
 For more information, see [Delegation](delegation.md).
 
-## Classes and interfaces practice
+## Practice
 
 ### Exercise 1 {initial-collapse-state="collapsed" collapsible="true" id="classes-interfaces-exercise-1"}
 

docs/topics/tour/kotlin-tour-intermediate-libraries-and-apis.md

@@ -174,7 +174,7 @@ fun main() {
 
 This is the easiest way to opt in, but there are other ways. To learn more, see [Opt-in requirements](opt-in-requirements.md).
 
-## Libraries and APIs practice
+## Practice
 
 ### Exercise 1 {initial-collapse-state="collapsed" collapsible="true" id="libraries-exercise-1"}
 

docs/topics/tour/kotlin-tour-intermediate-null-safety.md

@@ -359,7 +359,7 @@ safer by stopping as soon as a `null` value or invalid case is detected.
 
 For more information about how you can use `return` in your code, see [Returns and jumps](returns.md).
 
-## Null safety practice
+## Practice
 
 ### Exercise 1 {initial-collapse-state="collapsed" collapsible="true" id="null-safety-exercise-1"}
 

docs/topics/tour/kotlin-tour-intermediate-objects.md

@@ -160,7 +160,7 @@ at this point. The `getBongs()` function is called with parameter `12`.
 
 For more information, see [](object-declarations.md#companion-objects).
 
-## Object declarations practice
+## Practice
 
 ### Exercise 1 {initial-collapse-state="collapsed" collapsible="true" id="objects-exercise-1"}
 

docs/topics/tour/kotlin-tour-intermediate-open-special-classes.md

@@ -395,7 +395,7 @@ This can significantly reduce memory footprint and improve your code's runtime p
 
 For more information about inline value classes, see [Inline value classes](inline-classes.md).
 
-## Special classes practice
+## Practice
 
 ### Exercise 1 {initial-collapse-state="collapsed" collapsible="true" id="special-classes-exercise-1"}