Update best-practices/sdk-principles/kotlin.md

Co-authored-by: Umair <157486912+umair-ably@users.noreply.github.com>

Author: ttypic

best-practices/sdk-principles/kotlin.md

@@ -6,7 +6,7 @@ Kotlin is the primary development language for any JVM-based SDKs (Java, Scala,
 
 ## Core SDK
 
-The Core SDK serves is a Pub/Sub channel SDK without additional plugins like VcDiff, LiveObjects, or Push Notifications.
+The Core SDK serves as a Pub/Sub channel SDK without additional plugins like VcDiff, LiveObjects, or Push Notifications.
 
 Our current pub/sub channel SDK should be split into several modules: