Fixed incorrect substring usage in find

[nilsms]

When using substring in fnc_find with initialIndex, the substring should be from the index and to the end of the haystack.
The original code would return -1 for the example:
_result = ["frog-headed fish", "f", 5] call CBA_fnc_find;
And not 12 as shown in the example.

[Killswitch00]

Please update the function tests as well, with the sample code from the function header (strings\test_strings.sqf, where CBA_fnc_find is tested):

_pos = ["frog-headed fish", "f", 5] call CBA_fnc_find;
TEST_OP(_pos,==,12,_fn);

[Killswitch00]

Thanks. Merging.