[fuzz] remove \L in sed regex for filenames in gen_target.sh

[fuzz] remove \L in sed regex for filenames in gen_target.sh

Summary:
on `master` branch, if i ran `gen_target.sh` i would get the following output:
https://gist.github.com/savil/2b3114eaf34c3ef8499b65005e374841

Without it, I get the files generated that are already checked-in (as expected).

Looking at the sed docs (https://www.gnu.org/software/sed/manual/html_node/Regular-Expressions.html) i don't see an explanation for what `\L` may do in whatever regex sed follows. Does this look correct? cc @yuntai