libavutil/opt.h: Fix duplicate words

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/opt.h

@@ -163,7 +163,7 @@
  *
  * @subsection avoptions_implement_named_constants Named constants
  *      It is possible to create named constants for options. Simply set the unit
- *      field of the option the constants should apply to to a string and
+ *      field of the option the constants should apply to a string and
  *      create the constants themselves as options of type AV_OPT_TYPE_CONST
  *      with their unit field set to the same string.
  *      Their default_val field should contain the value of the named