Fix <overlay-config-signature> comments

Fix comments that refer to <overlay-config-signature> as
<config-signature>.

Bug: none
Test: build
Change-Id: I1d3e5e66ec3da993082f3441997b0d243d61520b
diff --git a/cmds/idmap2/libidmap2/ResourceMapping.cpp b/cmds/idmap2/libidmap2/ResourceMapping.cpp
index 122f068..31f1c16 100644
--- a/cmds/idmap2/libidmap2/ResourceMapping.cpp
+++ b/cmds/idmap2/libidmap2/ResourceMapping.cpp
@@ -71,9 +71,9 @@
   if (!target_package.DefinesOverlayable()) {
     return (sDefaultPolicies & fulfilled_policies) != 0
                ? Result<Unit>({})
-               : Error(
-                     "overlay must be preinstalled or signed with the same signature as the "
-                     "target");
+               : Error("overlay must be preinstalled, signed with the same signature as the target,"
+                       " or signed with the same signature as the package referenced through"
+                       " <overlay-config-signature>.");
   }
 
   const OverlayableInfo* overlayable_info = target_package.GetOverlayableInfo(target_resource);