Fix an obsoletion misconfig that inadvertently blocks use of Layout/FirstParameterIndentation

[maxh]

In April 2019, "Add IndentFirstArgument and IndentFirstParameter" #6982 forked the existing FirstParameterIndentation cop into two and added new argument-related functionality.

A few weeks ago, these two cops were renamed as part of broad renaming effort (#7077), and it just so happened IndentFirstParameter now has the name of the old pre-forked cop.

https://github.com/rubocop-hq/rubocop/pull/7468/files#diff-a19f978b95beb4caeb1831d6bef7e443R11

The obsoletion config now thinks the two forked cops are the same, but they are not. When I try to run RuboCop locally from master against our codebase, I get this spurious warning:

Error: The `Layout/FirstParameterIndentation` cop has been renamed to `Layout/FirstArgumentIndentation`.
(obsolete configuration found in .rubocop.yml, please update it)

This PR should fix that.

---

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[bbatsov]

Thanks!