Update tag.go

[rocktavious]

Problem

The current description for the --assign is hard to understand

Solution

Rip the description directly from the docs which is much clearer - https://docs.opslevel.com/docs/graphql#assigning-tags-to-a-

Checklist

• I have run this code, and it appears to resolve the stated issue.
• This PR has no user interface changes or has already received approval from product management to change the interface.
• Make a changie entry that explains the customer facing outcome of this change

[rdohms]

Much clearer, appreciate it.