fix: Correct typos and formatting in the README.md file.

[YouMingYeh]

• Fixed typo in the Cargo.toml example: removed the stray extra comma so the tauri-plugin-python entry now reads

  tauri-plugin-python = { version = "0.3", features = ["pyo3"] }

• Added missing colon in the Python function declaration in src-tauri/src-python/main.py examples:

  def greet_python(rust_var):
      return str(rust_var) + " from python"

• Tidied up surrounding comments and spacing for consistency.

No behavior changes; documentation only.