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 042d6f3 (doc:
git-checkout: clarify `-b` and `-B`, 2025-09-10) that this was probably
an unintended mix-up.

Acked-by: Julia Evans <julia@jvns.ca>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: LemmingAvalanche

Documentation/git-checkout.adoc

@@ -61,7 +61,7 @@ uncommitted changes.
 `git checkout -B <branch> [<start-point>]`::
 
 	The same as `-b`, except that if the branch already exists it
-	resets `_<branch>_` to the start point instead of failing.
+	resets _<branch>_ to the start point instead of failing.
 
 `git checkout --detach [<branch>]`::
 `git checkout [--detach] <commit>`::
@@ -155,7 +155,7 @@ of it").
 
 `-B <new-branch>`::
 	The same as `-b`, except that if the branch already exists it
-	resets `_<branch>_` to the start point instead of failing.
+	resets _<branch>_ to the start point instead of failing.
 
 `-t`::
 `--track[=(direct|inherit)]`::