Fix installation for Ubuntu in README.md

[shaobosong]

• PR Description If '/usr/local/bin' does not exist, 'install' will eventually result in a regular file named '/usr/local/bin' being created.

• Please check if the PR fulfills these requirements

• [ ] Cheatsheets are up-to-date (run go generate ./...)

• [ ] Code has been formatted (see here)

• [ ] Tests have been added/updated (see here for the integration test guide)

• [ ] Text is internationalised (see here)

• [ ] If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)

• [ ] Docs have been updated if necessary

• [x] You've read through your own file changes for silly mistakes etc

[mark2185]

LGTM

[mark2185]

If you could squash your commits, that would be great. There's no need for both of them.