Bump the version of sbt in the tutorial. (#580)

Author: blakehawkins

doc/tutorial/basic/index.md

@@ -45,7 +45,7 @@ scalaJSUseMainModuleInitializer := true
 Last, we need a `project/build.properties` to specify the sbt version (you can find the latest version [here](https://www.scala-sbt.org/download.html)):
 
 {% highlight scala %}
-sbt.version=1.3.7
+sbt.version=1.7.1
 {% endhighlight %}
 
 That is all we need to configure the build.