ftdetect: Use setf FALLBACK to set Cargo.toml filetype (#311)

vim-toml recently started using `setf`.[1] As a result, the filetype set
by rust.vim is no longer overridden. Use `setf FALLBACK` as this is the
proper mechanism for uncertain filetypes.

[1]: cespare/vim-toml@d257faa

Author: heftig

ftdetect/rust.vim

@@ -1,6 +1,6 @@
 " vint: -ProhibitAutocmdWithNoGroup
 
 autocmd BufRead,BufNewFile *.rs setf rust
-autocmd BufRead,BufNewFile Cargo.toml if &filetype == "" | set filetype=cfg | endif
+autocmd BufRead,BufNewFile Cargo.toml setf FALLBACK cfg
 
 " vim: set et sw=4 sts=4 ts=8: