doc: git-checkout: fix placeholder markup

doc: git-checkout: fix placeholder markup

The placeholder markup is underscore (_), not backtick (`) as well.

The inline-verbatim markup (backticks) handle interior formatting. This
means in this case that it applies HTML `<code>` to the underscores and
`<em>` to the placeholder.

That is the effect, anyway; we can see from the rest of 042d6f34 (doc:
git-checkout: clarify `-b` and `-B`, 2025-09-10) that this was probably
an unintended mix-up.

Acked-by: Julia Evans <redacted>
Signed-off-by: Kristoffer Haugsbakk <redacted>
Signed-off-by: Junio C Hamano <redacted>