Update README overview with new plugin name

[bananabele]

The overview section of the README (here and in Bonsai) has two names for the plugin:
Sensu Go Metric Tag Enrichment is a Sensu Go plugin for enriching metric points with Entity metadata (e.g. entity name, labels) and Check metadata (e.g. check name, labels), adding them as Metric Tags. The Sensu Metric Tag Mutator is useful for adding context to generic metrics collected from plugins that generate metrics in formats that don't support tags (e.g. Nagios Perfdata).

This issue is to update the second term ("The Sensu Metric Tag Mutator") to "Sensu Go Metric Tag Enrichment"