Update example build.sc with current library versions (#3055)

This avoids the build error:

```
1 targets failed
resolvedIvyDeps 
Resolution failed for 1 modules:
--------------------------------------------
  com.lihaoyi:scalatags_3:0.8.2 
	not found: /home/cosmin/.ivy2/local/com.lihaoyi/scalatags_3/0.8.2/ivys/ivy.xml
	not found: https://repo1.maven.org/maven2/com/lihaoyi/scalatags_3/0.8.2/scalatags_3-0.8.2.pom

--------------------------------------------

For additional information on library dependencies, see the docs at
https://mill-build.com/mill/Library_Dependencies.html
```

Pull request: #3055

example/basic/1-simple-scala/build.sc

@@ -3,8 +3,8 @@ import mill._, scalalib._
 object foo extends RootModule with ScalaModule {
   def scalaVersion = "2.13.11"
   def ivyDeps = Agg(
-    ivy"com.lihaoyi::scalatags:0.8.2",
-    ivy"com.lihaoyi::mainargs:0.4.0"
+    ivy"com.lihaoyi::scalatags:0.12.0",
+    ivy"com.lihaoyi::mainargs:0.6.2"
   )
 
   object test extends ScalaTests {
@@ -120,4 +120,4 @@ error: Missing argument: --text <str>
 // The most common *tasks* that Mill can run are cached *targets*, such as
 // `compile`, and un-cached *commands* such as `foo.run`. Targets do not
 // re-evaluate unless one of their inputs changes, whereas commands re-run every
-// time.
+// time.