Fix typo in "Skip packageDoc" section (#1705)

Setting needs to be set in the `Compile` configuration, not the `compile` task.

Author: esamson

src/sphinx/formats/universal.rst

@@ -323,7 +323,7 @@ this behaviour, add this to your ``build.sbt``
 
 .. code-block:: scala
 
-    compile / packageDoc / mappings := Seq()
+    Compile / packageDoc / mappings := Seq()
 
 Source `issue 651`_.