kotlin.md: use upsource:// link (IJSDK-77)
kotlin.md: use upsource:// link (IJSDK-77)

JetBrains/intellij-sdk-docs,JetBrains/intellij-sdk-docs,artspb/intellij-sdk-docs,artspb/intellij-sdk-docs,artspb/intellij-sdk-docs,JetBrains/intellij-sdk-docs,artspb/intellij-sdk-docs