fix sbt version in getting-started.md

https://github.com/lampepfl/dotty/blob/306e4fcffeac990d8edf5c49ae2b6e49a79dac39/sbt-dotty/src/dotty/tools/sbtplugin/DottyPlugin.scala#L169

Author: xuwei-k

docs/docs/usage/getting-started.md

@@ -10,7 +10,7 @@ title: Getting Started: Users
 This is an easy way to try Dotty without installing anything, directly in your browser.
 
 ### sbt
-The fastest way to create a new project compiled by Dotty is using [sbt (1.1.4+)](http://www.scala-sbt.org/)
+The fastest way to create a new project compiled by Dotty is using [sbt (1.3.6+)](http://www.scala-sbt.org/)
 
 Create a simple Dotty project:
 ```bash