html: Fix example blocks - escape html

While example blocks do not render inline markup and are thus parsed raw in
some way, their contents are not literal html and thus still need to be html
escaped.

org/testdata/blocks.org

@@ -25,6 +25,8 @@ it also has multiple parameters
 src, example & export blocks treat their content as raw text
 /inline/ *markup* is ignored
       and whitespace is honored and not removed
+
+content of example blocks is still html escaped - see <script>alert("escaped")</script>
 #+END_EXAMPLE
 
 : examples like this