fix warnings in build.sbt

[xuwei-k]

flyway-sbt/build.sbt:18: warning: method in in trait ScopingSetting is deprecated (since 1.5.0): `in` is deprecated; migrate to slash syntax - https://www.scala-sbt.org/1.x/docs/Migrating-from-sbt-013x.html#slash
      scalacOptions in (Compile, doc) ++= {
                    ^
flyway-sbt/build.sbt:21: warning: method in in trait ScopingSetting is deprecated (since 1.5.0): `in` is deprecated; migrate to slash syntax - https://www.scala-sbt.org/1.x/docs/Migrating-from-sbt-013x.html#slash
          (baseDirectory in LocalRootProject).value.getAbsolutePath,
                         ^