Update first_test.md (#12)

Author: Wiicart

docs/en/user_guide/introduction/first_test.md

@@ -32,7 +32,7 @@ for running tests automatically.
 :::
 
 ::: warning
-You're class can't be `final`. This is because MockBukkit uses reflection to
+Your class can't be `final`. This is because MockBukkit uses reflection to
 modify the behavior of the class under test. In Kotlin, classes are `final` by
 default,
 so you must declare them as `open` for MockBukkit to function properly.