pdh_check has a typo causing tags to not work as intended

[nvecellio]

https://github.com/DataDog/integrations-core/blob/master/pdh_check/check.py#L65

                if not counter.is_single_instance():
                    tag = "instance=%s" % key
                    tags.append(tag)

The "=" character causes tags to show in the UI as just a value, and not a key:value, making it more difficult to graph these metrics. A simple change to ":" causes these tags to work as expected.

[derekwbrown]

Was resolved in #927

[derekwbrown]

Integration fixed with proper tagging in PR #927