[Python-checkins] devguide (hg_transition): Typo spotted by Sandro Tosi

[Terry Reedy]

>   This will check and/or fix various common things people forget to do for
> -patches, such as adding any new files needing for the patch to work (do not
> +patches, such as adding any new files needing for the patch to work (note

/needing/needed/