Tweak Seq migration guide

Author: dwijnand

_overviews/core/collections-migration-213.md

@@ -115,7 +115,7 @@ types.  When cross-building your library for Scala 2.12 and 2.13 this could eith
 - continuing to use `scala.Seq` which means it stays source and binary-compatible in 2.12, but would have to
     have immutable sequence semantics (but that might already be the case).
 - switch to explicitly using immutable Seq in both Scala 2.12 and 2.13, which means breaking source, binary and
-    semantic compatibility in 2.12:
+    (possibly) semantic compatibility in 2.12:
 
 ~~~ scala
 import scala.collection.{ immutable => sci }