fix: suggest git switch over checkout

The switch command has been around for 4 years now, and while still
marked as experimental, I think it's fine to "switch" to it in the
suggestion for local cherry-picking. It's unlikely that users are using
such old git versions that it doesn't support git switch --create.

src/backport.ts

@@ -417,7 +417,7 @@ export class Backport {
       git fetch origin ${target}
       git worktree add -d .worktree/${branchname} origin/${target}
       cd .worktree/${branchname}
-      git checkout -b ${branchname}
+      git switch --create ${branchname}
       git cherry-pick -x ${commitShasToCherryPick.join(" ")}
       \`\`\``;
   }