[gdb/python] Fix doc string of valpy_const_value

In gdb/python/py-value.c, in the value_object_methods array I noticed:
...
  { "const_value", valpy_const_value, METH_NOARGS,
    "Return a 'const' qualied version of the same value." },
...

Fix the qualied -> qualified typo.

Reviewed-By: Tom Tromey <tom@tromey.com>