Fix migration tool installation instructions

Author: MaximeKjaer

README.md

@@ -43,7 +43,6 @@ The migration rules use scalafix. Please see the [official installation instruct
 
 ```scala
 // project/plugins.sbt
-
 addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.9.8")
 ```
 
@@ -54,6 +53,7 @@ The `Collection213Upgrade` rewrite upgrades to the 2.13 collections without the
 ```scala
 // build.sbt
 scalafixDependencies in ThisBuild += "org.scala-lang.modules" %% "scala-collection-migrations" % "2.1.1"
+addCompilerPlugin(scalafixSemanticdb)
 scalacOptions ++= List("-Yrangepos", "-P:semanticdb:synthetics:on")
 ```
 
@@ -71,8 +71,9 @@ To cross-build for 2.12 and 2.11, the rewrite rule introduces a dependency on th
 
 ```scala
 // build.sbt
-scalafixDependencies in ThisBuild += "org.scala-lang.modules" %% "scala-collection-migrations" % "2.1.2"
+scalafixDependencies in ThisBuild += "org.scala-lang.modules" %% "scala-collection-migrations" % "2.1.1"
 libraryDependencies +=  "org.scala-lang.modules" %% "scala-collection-compat" % "2.1.2"
+addCompilerPlugin(scalafixSemanticdb)
 scalacOptions ++= List("-Yrangepos", "-P:semanticdb:synthetics:on")
 ```