Document Doxygen issue fix.

https://github.com/doxygen/doxygen/issues/6456 is fixed in
Doxygen 1.8.15. Update teh release checklist to reflect this

Thanks to derf for checking this.

doc/release.txt

@@ -41,11 +41,8 @@ Win32 binaries:
   - ./configure --host=i686-w64-mingw32 --prefix=/path/to/builddir/mingw \
         PKG_CONFIG_PATH=/path/to/builddir/mingw/lib/pkgconfig
   - make && make check && make -C doc/latex
-    - Doxygen 1.18.13 fails because of unescaped '#' characters in URLs
-      (and escaping them breaks the HTML output).
-      But at least two years old: https://github.com/doxygen/doxygen/issues/6456
-      Doxygen 1.8.3 works.
-- Add api docs to opus-codec.org.
+    - If Doxygen fails because of unescaped '#' characters in URLs
+      Update to at least Doxygen 1.8.15. Doxygen 1.8.3 also works.
 - mkdir opusfile-${version}-win32
 - Copy AUTHORS COPYING README.md include/opusfile.h to the release dir.
   - Don't put opusfile.h in an opusfile-${version}-win32/include directory,