embedding : add --pooling option to README.md [no ci] (#8934)

This commit adds the `--pooling` option to the README.md file in the
`examples/embedding` directory.

The motivation for adding this options is that currently if the model
used does not specify a pooling type the embedding example will fail
with the following error message:
```console
main: error: pooling type NONE not supported
```

This commit also updates the name of the executable in the examples
section.