Update 3-forecast.md

[runofthemill]

Add needed --start-date flag to "Forecasting multiple providers" command.

Without the flag, no report is generated, and the log file includes a warning that no jobs were transformed:

# Logfile created on 2024-10-09 22:46:39 +0000 by logger.rb/v1.6.0
I, [2024-10-09T22:46:39.075648 #1]  INFO -- : Using GitHub Features: Defaults
I, [2024-10-09T22:46:39.075696 #1]  INFO -- : Forecasting '/data/tmp/forecast/jobs/10-09-2024-22-44_jobs_0.json'
W, [2024-10-09T22:46:39.084369 #1]  WARN -- : No jobs transformed