Fix Ticker @implicitNotFound message to reference testkit-available API

The previous message pointed users at `ticked { implicit ticker => }`, which
is defined in the internal tests module (tests/shared/src/test/scala/cats/
effect/Runners.scala) and is not visible to downstream users of the public
testkit artifact. Replace it with an instruction that uses the constructor
already exported from testkit.

Closes #1875.

Author: mvanhorn

testkit/shared/src/main/scala/cats/effect/testkit/TestInstances.scala

@@ -253,7 +253,7 @@ trait TestInstances extends ParallelFGenerators with OutcomeGenerators with Sync
   }
 
   @implicitNotFound(
-    "could not find an instance of Ticker; try using `in ticked { implicit ticker =>`")
+    "could not find an instance of Ticker; try declaring one with `implicit val ticker: Ticker = Ticker()`")
   case class Ticker(ctx: TestContext = TestContext())
 }