runtime(doc): Highlight the error message at *E1513*

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/message.txt b/runtime/doc/message.txt
index 5848074..6e1122d 100644
--- a/runtime/doc/message.txt
+++ b/runtime/doc/message.txt
@@ -123,7 +123,7 @@
 path leading to the file.
 
 							*E1513* >
-  Cannot edit buffer. 'winfixbuf' is enabled
+  Cannot edit buffer. 'winfixbuf' is enabled ~
 
 If a window has 'winfixbuf' enabled, you cannot change that window's current
 buffer. You need to set 'nowinfixbuf' before continuing. You may use [!] to