Change {{ to avoid markdown warning

We changed 2013/hou/doc/example.markdown to deal with this
github warning:

    Your site is having problems building: The variable {{ on line 36 in 2013/hou/doc/example.markdown was not properly closed with }}. For more information, see https://docs.github.com/github/working-with-github-pages/troubleshooting-jekyll-build-errors-for-github-pages-sites#tag-not-properly-terminated.

2013/hou/doc/example.markdown

@@ -33,7 +33,7 @@ You can directly feed it to the renderer. If you're wondering about what the fol
 	iiEJpp%
 	uuEJpp%
 	ZnY|{|e
-	[mZ|{{#
+	[mZ|\{\{#
 	[mZ|||f
 	kkdr{{%
 	lkdr{{>
@@ -349,4 +349,4 @@ Continue to the EOF
 
 The last line, well, is the output filename. Just remember to throw in two ']'s and don't put a newline after it.
 
-	]]example.ppm
+	]]example.ppm