[gdb/guile] Fix doc string for value-optimized-out?

In gdb/guile/scm-value.c, I noticed in the value_functions array initializer:
...
  { "value-optimized-out?", 1, 0, 0,
    as_a_scm_t_subr (gdbscm_value_optimized_out_p),
    "\
Return #t if the value has been optimizd out." },
...
There's a typo in the doc string.

Fix this by using "optimized".

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