fix(variable): drop --scope from the group example in update

What does this MR do and why?

The last example in glab variable update --help cannot be run:

$ cat token.txt | glab variable update GROUP_TOKEN -g mygroup --scope=prod
ERROR
Scope is not required for group variables.

The command validates that combination a few lines below the example:

if cmd.Flags().Changed("scope") && o.group != "" {
	return cmdutils.FlagError{Err: errors.New("scope is not required for group variables")}
}

and its own Long text already says "The --scope flag does not apply to group variables". So the example contradicts both the description and the validation in the same file.

Dropping --scope=prod leaves a valid group variable update, which I confirmed reaches the API instead of being rejected up front.

docs/source/variable/update.md is regenerated in the same commit.

Scope note

glab variable set carries the same example text, but it has no equivalent validation, so its version does run. I have left set alone: whether it should also reject -g with --scope is a behavioural decision for the maintainers, not something to change as part of a documentation fix.

Test coverage

None added. This is a one-line example correction, and the failure is reproducible directly from the help text; the existing tests for this command are unaffected.

Related issues

Closes #8441 (closed)