Replace .text with .json() in garmin.get_hrv_data()

[psdupvi]

Related to https://github.com/cyberjunky/python-garminconnect/issues/117 which I just opened -- garmin.get_hrv_data() has a last line of

return self.modern_rest_client.get(url).text #.json()

instead of

return self.modern_rest_client.get(url).json()

[cyberjunky]

@psdupvi Good find! Thanks!