kernel-doc: properly document array arguments of function

Documentation for array parameters passed in a function, like the first
argument in the function below, weren't getting exported in the rst
format, although they work fine for html and pdf formats:

  void drm_clflush_pages(struct page * pages[], unsigned long num_pages)

That's because the string key to store the description in the
parameterdescs dictionary doesn't have the [] suffix.  This cleans up
the suffix from the key before accessing the dictionary.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Fixes: c0d1b6ee ("kernel-doc: produce RestructuredText output")
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>