Fix wrong asterisk use in Lint/UselessRuby2Keywords document

[r7kamura]

I found a problem with the UselessRuby2Keywords cop documentation page not rendered well.

• https://docs.rubocop.org/rubocop/1.34/cops_lint.html#lintuselessruby2keywords

Before

Look around the <strong> element.

<p><code>ruby2_keywords</code> should only be called on methods that accept an argument splat
(<code><strong>args</code>) but do not explicit keyword arguments (<code>k:</code> or <code>k: true</code>) or
a keyword splat (<code></strong>*kwargs</code>).</p>

After

<p><code>ruby2_keywords</code> should only be called on methods that accept an argument splat
(<code>*args</code>) but do not explicit keyword arguments (<code>k:</code> or <code>k: true</code>) or
a keyword splat (<code>**kwargs</code>).</p>

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