Fix rustc-josh-sync push command argument order in README

[Binlogo]

This pull request makes a minor update to the documentation in README.md to clarify the usage of the rustc-josh-sync push command. The order of the <branch> and <your-github-username> arguments has been corrected to reflect the actual command syntax.

Code reference:

    /// Push changes into the main `rust-lang/rust` repository `branch` of a `rustc` fork under
    /// the given GitHub `username`.
    /// The pushed branch should then be merged into the `rustc` repository.
    Push {
        /// Path to the josh-sync TOML config file.
        #[clap(long, default_value(DEFAULT_CONFIG_PATH))]
        config_path: PathBuf,

        /// Path to a file storing the last synchronized rustc commit.
        #[clap(long, default_value(DEFAULT_RUST_VERSION_PATH))]
        rust_version_path: PathBuf,

        /// Branch that should be pushed to your remote
        branch: String,

        /// Your GitHub usename where the fork is located
        username: String,

        /// Print executed commands.
        #[clap(long, short = 'v')]
        verbose: bool,
    },

[Kobzol]

Huh, I thought I fixed this already, but it was probably in another repo. Thank you!