Fixes errors caused by copytext being a shit

Basically, copytext's first argument is inclusive, but its 2nd argument isn't.

so copytext("123456789", 2, 8) returns 234567.

When I googled "byond copytext" (to see how it worked in byond and if it used 1 or 0 based indexing) the first link is http://www.byond.com/docs/guide/chap15.html and this does nothing to spell this out.

The reference guide (that i should have used) spells this out more clearly, but it is confusing none the less: http://www.byond.com/docs/ref/info.html#/proc/copytext
Merge pull request #4526 from MrStonedOne/thirdtimesthecharm

Fixes errors caused by copytext being a shit