Correct default MetricExporterIntervalMilliseconds from 20000 to 60000 in documentation (open-telemetry#2315)

Author: haipz

src/OpenTelemetry.Exporter.Geneva/README.md

@@ -340,7 +340,7 @@ the `ConnectionString`.
 #### `MetricExportIntervalMilliseconds` (optional)
 
 Set the exporter's periodic time interval to export Metrics. The default value
-is 20000 milliseconds.
+is 60000 milliseconds.
 
 #### `PrepopulatedMetricDimensions` (optional)