RoR: Update link in Active Record Associations

[WongYC-66]

Because

Ruby on rails official guide has structure change from v7.1 to v7.2

This is about assignment 3.

Skim chapter 4. It contains all the methods that you gain by using various associations. You certainly don’t need to memorize the whole list, but poke through it. You’ll end up using most of them.

• in v7.1, it refers to 4 Detailed Association Reference
• in v7.2, it refers to 4 Advanced Associations

I believe the assignment is referring to v7.1 #section 4

This PR

• Update a link in assignment, to use guide of v7.1

Issue

Closes #XXXXX

Additional Information

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[JoshDevHub]

I would prefer to change the instructions in the assignment to fit the new structure of the guide rather than backlink to an older version.

Changing the instructions to something like this would work for getting all the same information that the assignment did with the previous structure of the guide:

1. Read the [Rails Guide on Associations](https://guides.rubyonrails.org/association_basics.html). Start by skimming chapters 1 and 2 (which you should have already done).
2. Read all the content in chapters 3, 4, and 7.

[WongYC-66]

Updated as @JoshDevHub suggested.