coda: Fix typo in the struct CodaCred documentation

Documentation mistakenly refers to a different type while explaining
the contents of the struct CodaCred.

Fix the typo in the struct CodaCred description in the documentation.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>