Update example in readme for convert_sum_to_gauge and convert_gauge_t…

…o_sum (open-telemetry#35022)

**Description:** The feature gate
"processor.transform.ConvertBetweenSumAndGaugeMetricContext" was updated
from Alpha to Beta. This means that it is now enabled by default. The
example in the readme should match with the default behavior which is
that the functions convert_sum_to_gauge and convert_gauge_to_sum must be
used on the "metric" context

**Link to tracking Issue:** open-telemetry#34567

Maybe the Readme should state clearly in which context the functions can
be used?

processor/transformprocessor/README.md

@@ -130,12 +130,12 @@ transform:
     - context: metric
       statements:
         - set(description, "Sum") where type == "Sum"
+        - convert_sum_to_gauge() where name == "system.processes.count"
+        - convert_gauge_to_sum("cumulative", false) where name == "prometheus_metric"
     - context: datapoint
       statements:
         - limit(attributes, 100, ["host.name"])
         - truncate_all(attributes, 4096)
-        - convert_sum_to_gauge() where metric.name == "system.processes.count"
-        - convert_gauge_to_sum("cumulative", false) where metric.name == "prometheus_metric"
         
   log_statements:
     - context: resource