Simply put, "*.doc". However, many Windows types of files have spaces in them, so you need to be careful.
You can't just do: mv *.doc *.txt, as that doesn't make sense. What you want to do is a little shell command like:
for i in *.doc ; do
n=`basename "$i" .doc`
mv "$i" "$n".txt
Note that the quote marks are required if your filenames have spaces in them.