docs: fix the branch example for lazy.nvim

[frantisekstanko]

Description

Fix a small, but confusing typo in the README.

The previous values were incorrect. The new values reflect the correct way of using the branch, as showcased in the previous examples.

Type of change

• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project (stylua)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (lua annotations)

[jamestrew]

Thanks