Fix: Doc-runner uses wrong file extension.

Updated Doc-runner so that it converts files with
the md-extension instead of the txt-extension.

refs #4214

bin/createdoc.sh

@@ -1,7 +1,7 @@
 #!/bin/sh
 
-for filepath in `find ../doc -type f -name "*.txt"`; do
-    markdown $filepath > `dirname $filepath`/`basename $filepath .txt`.html
+for filepath in `find ../doc -type f -name "*.md"`; do
+    markdown $filepath > `dirname $filepath`/`basename $filepath .md`.html
 done
 
 exit 0