[Taler] libeufin documentation correction

[Slack Coder]

Hi,

There is a mistake in the documentation for [building libeufin-bank from source](https://docs.taler.net/libeufin/bank-manual.html#building-from-source).

    ```
    ./bootstrap
    ./configure --prefix=$PREFIX
    make install-bank
    ```

The is no 'install-bank' make target, it should be

    ```
    ./bootstrap
    ./configure --prefix=$PREFIX
    make
    make install-nobuild-bank
    ```

Thanks!

Slack Coder