docs(subprocess): Fix documentation of imported class

Author: tony

docs/internals/subprocess.md

@@ -9,6 +9,6 @@
      close_fds, shell, cmd, env, text, universal_newlines,
      startupinfo, creationflags, restore_signals, start_new_session,
      group, extra_groups, user, umask, pass_fds, encoding, errors,
-     run
+     run, SkipDefaultFieldsReprMixin
 
 ```