Fix typo in comment

Author: dmerejkowsky

src/libstd/sys/unix/fd.rs

@@ -282,7 +282,7 @@ impl Drop for FileDesc {
         // reason for this is that if an error occurs we don't actually know if
         // the file descriptor was closed or not, and if we retried (for
         // something like EINTR), we might close another valid file descriptor
-        // (opened after we closed ours.
+        // opened after we closed ours.
         let _ = unsafe { libc::close(self.fd) };
     }
 }