Update coroutines-guide.md link (#2661)

Author: koshachy

docs/topics/coroutines-guide.md

@@ -32,5 +32,5 @@ In order to use coroutines as well as follow the examples in this guide, you nee
 ## Additional references
 
 * [Guide to UI programming with coroutines](https://github.com/Kotlin/kotlinx.coroutines/blob/master/ui/coroutines-guide-ui.md)
-* [Coroutines design document (KEEP)](https://github.com/Kotlin/kotlin-coroutines/blob/master/kotlin-coroutines-informal.md)
+* [Coroutines design document (KEEP)](https://github.com/Kotlin/KEEP/blob/master/proposals/coroutines.md)
 * [Full kotlinx.coroutines API reference](https://kotlin.github.io/kotlinx.coroutines)