Fix error in insertion point logic

Author: mpilquist

core/shared/src/main/scala/fs2/Chunk.scala

@@ -978,7 +978,7 @@ object Chunk
           chunks(j + 1)(0)
         } else {
           // The requested index is not an exact match but located in the chunk after the returned insertion point
-          val k = -j + 1
+          val k = -(j + 1)
           val accLenBefore = if (k == 0) 0 else lengths(k - 1)
           chunks(k)(i - accLenBefore)
         }