doc: develop: commands: Fix function prototype

When using the previous prototype you got a compiler warning like this:

    warning: initialization of 'int (*)(struct cmd_tbl *, int,  int,  char * const*)' from incompatible pointer type 'int (*)(struct cmd_tbl *, int,  int,  const char **)' [-Wincompatible-pointer-types]

Fixes: 3d9640f5 ("doc: expand README.commands")
Signed-off-by: Alexander Dahl <ada@thorsis.com>