README: backquote some text in changes list for 1.1.12

The README file is meant to be valid Markdown.

The changes listed for version 1.1.12 included:

    do not choke on <br></br> etc.

Of course, that caused a "<br />" tag to end up in the
rendered output.

Quote the tags with backquotes like so:

    do not choke on `<br></br>` etc.

This makes it render as intended in the xhtml output.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>

README

@@ -194,7 +194,7 @@ Version History
 
 * clean up closing tag whitespace
 
-* do not choke on <br></br> etc.
+* do not choke on `<br></br>` etc.
 
 * add limited tilde-delimited code block support