useful_links.md: fix "IntelliJ Platform Artifacts Repository" link
useful_links.md: fix "IntelliJ Platform Artifacts Repository" link

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