Update documentation of nuxeo-operation

The example at https://elements.nuxeo.com/elements/nuxeo-operation does not work because the params property does not use the proper quotes. It uses "simple quotes inside double quotes", it should be the reverse, using "double quotes inside simple quotes".
So it must be...

params='{"query": "select from Document"}'

...instead of:

params="{'query': 'select from Document'}"