Update opening_hours documentation

[mplsmitch]

This PR corrects an inconsistency in the documentation around opening_hours. @hbruch noted that in the documentation under Hours and Dates of Operation it says:

Hours and dates of operation SHOULD be published even in cases where services are continuously available 24/7.

but system_information.opening_hours is REQUIRED so the line quoted above should be changed from SHOULD to MUST.

This is kind of a gray area but my view is that this is an editorial change to the documentation and does not require a vote. If anyone disagrees with this we can put it to a vote. That would probably also make it a breaking change which would mean it would not take effect until v4.0. If no one has an issue it will go in the next minor version.

Is this a breaking change?

• [ ] Yes
• [ ] No
• [ X ] Unsure

Which files are affected by this change?

gbfs.md