Update the group by tags example in the README

Author: Enric Sala

README.md

@@ -130,7 +130,7 @@ result = influxdb.query('weather') ...
 % Another example with more options
 result = influxdb.query(TEST_TAGS) ...
     .fields('mean(temperature)', 'sum(rain)') ...
-    .groupByTags('city', 'station') ...
+    .groupByTags('country', 'city') ...
     .groupByTime('3h', 'linear') ...
     .limit(100) ...
     .execute();
@@ -142,9 +142,12 @@ The result of a query is an object that provides additional functionalities:
 % Check which series are present in a result
 series_names = result.names()
 
-% Extract a specific series
+% Get series with matching name
 weather = result.series('weather')
 
+% When grouping by tags, get series with matching tags
+weather_bcn = result.series('weather', 'city', 'barcelona')
+
 % Check which fields are present in a series
 field_names = weather.fields()