Docs: Update rules.md stop_times.txt best practices reference link

[mrajen27]

Updating the Rules.md documentation to fix the stop_times.txt best practices reference link 404 error.

In RULES.md, the link stop_times.txt best practices provided in the missing_timepoint_column notice is broken. Additionally, the Best Practice has no mention of timepoint, think link should refer to the specification instead.

It should be replaced by the following link:

https://gtfs.org/schedule/reference/#stop_timestxt

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[isabelle-dr]

Hello @mrajen27, thank you for fixing this issue so quickly! I left one comment requesting a change, besides this, it looks good 😊.

Another thing: could you please sign the CLA agreement.

Thanks for your participation in open source software!

[isabelle-dr]

@mrajen27, are you planning to continue working on this PR?

[isabelle-dr]

Closing, this is being solved in #1295. @mrajen27, we hope to see you around on other issues! 😊