unique: fix inconsistent panic prefix in canonmap cleanup path

Other panic messages in the file use the "unique.canonMap"
prefix, but this one used "internal/sync.HashTrieMap",
looks like it was copied from the "internal/sync" package.

Change-Id: Ic3e85154eb5a593d41c19013d5696afed2178b7e
Reviewed-on: https://go-review.googlesource.com/c/go/+/713660
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Sean Liao <[email protected]>
Auto-Submit: Sean Liao <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: David Chase <[email protected]>

Author: mohanson

src/unique/canonmap.go

@@ -232,7 +232,7 @@ func (m *canonMap[T]) cleanup(hash uintptr, wp weak.Pointer[T]) {
 			// which requires the parents' lock.
 			for i.parent != nil && i.empty() {
 				if hashShift == 8*goarch.PtrSize {
-					panic("internal/sync.HashTrieMap: ran out of hash bits while iterating")
+					panic("unique.canonMap: ran out of hash bits while iterating")
 				}
 				hashShift += nChildrenLog2