changes.src: to hide a keyword, %idefine foo $%??

The $ in "%idefine ptr $%??" was missing.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
diff --git a/doc/changes.src b/doc/changes.src
index 2a22fb3..35db4e1 100644
--- a/doc/changes.src
+++ b/doc/changes.src
@@ -70,7 +70,7 @@
    much more likely to indicate a MASM-ism encountered in NASM than it
    is a valid label.  This warning can be suppressed with \c{-w-ptr},
    the \c{[warning]} directive (see \k{opt-w}) or by the macro
-   definition \c{%idefine ptr %??}.
+   definition \c{%idefine ptr $%??}.
 
 \b When an error or a warning comes from the expansion of a multi-line
    macro, display the file and line numbers for the expanded macros.