Fix typo and add test

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -380,7 +380,7 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
    *  cause false unreachable warnings. See tests/patmat/t2425.scala
    *
    *  We cannot use type erasure here, as it would lose the constraints
-   *  invovling GADTs. For example, in the following code, type
+   *  involving GADTs. For example, in the following code, type
    *  erasure would loose the constraint that `x` and `y` must be
    *  the same type, resulting in false inexhaustive warnings:
    *

tests/patmat/i6197d.check

@@ -0,0 +1 @@
+5: Pattern Match Exhaustivity: _: Array[String]

tests/patmat/i6197d.scala

@@ -0,0 +1,7 @@
+def foo(x: Array[String]) = x match {
+  case _: Array[_] =>
+}
+
+def bar(x: Array[String]) = x match {
+  case _: Array[_ <: Int] =>
+}