Update functions.md
Update functions.md

Fix examples 2 and 3, replacing "first_" with "last_".
influxdata/docs.influxdata.com,influxdb/docs.influxdata.com,influxdata/docs.influxdata.com,influxdata/docs.influxdata.com,influxdb/docs.influxdata.com,influxdb/docs.influxdata.com,influxdata/docs.influxdata.com,influxdata/docs.influxdata.com