riched32: fix a compiler warning on clang

[Austin English]

-- 
-Austin
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diff --git a/dlls/riched32/tests/editor.c b/dlls/riched32/tests/editor.c
index 5cc269d..591821c 100644
--- a/dlls/riched32/tests/editor.c
+++ b/dlls/riched32/tests/editor.c
@@ -844,7 +844,7 @@ static void test_EM_POSFROMCHAR(void)
   /* Fails on builtin because horizontal scrollbar is not being shown */
   ok(pl.x < xpos ||
       broken(pl.x == xpos), /* Win9x, WinME and NT4 */
-      "EM_POSFROMCHAR reports x=%hd, expected value less than %d\n", pl.x, xpos);
+      "EM_POSFROMCHAR reports x=%u, expected value less than %d\n", pl.x, xpos);
   }
   DestroyWindow(hwndRichEdit);
 }