internal/lsp: fix error message for inlay hints

Fix the error message to describe inlay hints failure.

Change-Id: If4597bc3e513c4dce344f11f6fa92ba20e29681a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/411899
gopls-CI: kokoro <noreply+kokoro@google.com>
Run-TryBot: Suzy Mueller <suzmue@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

internal/lsp/lsp_test.go

@@ -977,7 +977,7 @@ func (r *runner) InlayHints(t *testing.T, spn span.Span) {
 	}))
 
 	if withinlayHints != got {
-		t.Errorf("format failed for %s, expected:\n%v\ngot:\n%v", filename, withinlayHints, got)
+		t.Errorf("inlay hints failed for %s, expected:\n%v\ngot:\n%v", filename, withinlayHints, got)
 	}
 }