use enumerate() instead of indexing

use enumerate() instead of indexing

Fixes:

warning: the loop variable `i` is used to index `time_data`
   --> src/rrd_graph_new.rs:388:14
    |
388 |     for i in 0..time_data.len() {
    |              ^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_range_loop
    = note: `#[warn(clippy::needless_range_loop)]` on by default
help: consider using an iterator and enumerate()
    |
388 |     for (i, <item>) in time_data.iter().enumerate() {
    |         ~~~~~~~~~~~    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>