use_file: Fix typos

Signed-off-by: Joe Richey <joerichey@google.com>

src/use_file.rs

@@ -41,7 +41,7 @@ pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
     Ok(())
 }
 
-// Returns the field descriptor for the device file used to retrieve random
+// Returns the file descriptor for the device file used to retrieve random
 // numbers. The file will be opened exactly once. All successful calls will
 // return the same file descriptor. This file descriptor is never closed.
 fn get_rng_fd() -> Result<libc::c_int, Error> {