Updated Install libffi7 package step
Hi @alankelly Can you please review this PR ? Thank you! Hi @alankelly Can you please review this PR ? Thank you! Hi @alankelly Can you please review this PR ? Thank you! Hi @alankelly Can you please review this PR ? Thank you!
While following the [TensorFlow Lite C++ minimal example](https://github.com/tensorflow/tensorflow/edit/master/tensorflow/lite/examples/minimal/README.md) instructions I encountered issue with `libffi7` package ([libffi.so.7: cannot open shared object file: No such file or directory](https://askubuntu.com/questions/1286772/libffi-so-7-cannot-open-shared-object-file-no-such-file-or-directory)) because since `Ubuntu 20.10` comes with `libff8` instead of `libffi7` and in TensorFlow Lite C++ minimal example it's still using `libffi7` package so we have to install `libffi7` by manually downloading the deb package from ubuntu focal (20.04) By following below steps so It's better to add below steps for Ubuntu users who are using `Ubuntu 20.10 or later `version to handle `libffi.so.7: cannot open shared object file: No such file or directory` error in our official documentation of [TensorFlow Lite C++ minimal example](https://github.com/tensorflow/tensorflow/edit/master/tensorflow/lite/examples/minimal/README.md) so please do the needful. Thank you!

```
1. wget http://es.archive.ubuntu.com/ubuntu/pool/main/libf/libffi/libffi7_3.3-4_amd64.deb
2. sudo dpkg -i libffi7_3.3-4_amd64.deb
```