Fix small mistake in Lua API docs

doc/lua_api.txt

@@ -1105,6 +1105,7 @@ minetest.pos_to_string({x=X,y=Y,z=Z}) -> "(X,Y,Z)"
 ^ Convert position to a printable string
 minetest.string_to_pos(string) -> position
 ^ Same but in reverse
+minetest.formspec_escape(string) -> string
 ^ escapes characters [ ] \ , ;  that can not be used in formspecs
 minetest.is_yes(arg)
 ^ returns whether arg can be interpreted as yes