lint: Advertise regal fix command

[charlieegan3]

Fixes #1014

This is the most simple implementation I could think of for this one!

…/regal regal-fix-prompt ➜ go run main.go lint package.rego

Rule:           opa-fmt
Description:    File should be formatted with `opa fmt`
Category:       style
Location:       package.rego:1:1
Text:           package p
Documentation:  https://docs.styra.com/regal/rules/style/opa-fmt

Rule:           use-rego-v1
Description:    Use `import rego.v1`
Category:       imports
Location:       package.rego:1:1
Text:           package p
Documentation:  https://docs.styra.com/regal/rules/imports/use-rego-v1

Rule:           use-assignment-operator
Description:    Prefer := over = for assignment
Category:       style
Location:       package.rego:3:3
Text:           x = 5
Documentation:  https://docs.styra.com/regal/rules/style/use-assignment-operator

1 file linted. 3 violations found.

Hint: 2/3 violations can be automatically fixed (use-rego-v1, use-assignment-operator)
      Run regal fix --help for more details.
exit status 3

[anderseknert]

We'll have to consider what to do here if we make the new directory-package-mismatch rule custom and not "default"... but let's just make a note on that for now.