Fix a typo in multiplatform.md (#3803)

Author: ValentineSokol

docs/topics/multiplatform/multiplatform.md

@@ -17,7 +17,7 @@ while retaining the flexibility and benefits of native programming.
 
 ### Android and iOS applications
 
-Sharing code between mobile platforms is a major Kotlin Multiplatform use cases. With Kotlin Multiplatform for mobile,
+Sharing code between mobile platforms is a major Kotlin Multiplatform use case. With Kotlin Multiplatform for mobile,
 you can build cross-platform mobile applications that share code between Android and iOS to implement networking,
 data storage and data validation, analytics, computations, and other application logic.