Update readme regarding enable.top (#469)

'enable.top' was renamed to 'collector.topmetrics'. This updates the README to reflect that change.

Author: arggh

README.md

@@ -41,7 +41,7 @@ the current version.
 |\-\-no-collector.diagnosticdata|Disable collecting metrics from getDiagnosticData||
 |\-\-no-collector.replicasetstatus|Disable collecting metrics from replSetGetStatus||
 |\-\-collector.dbstats|Enable collecting metrics from dbStats||
-|\-\-enable.top|Enable collecting metrics from top admin command||
+|\-\-collector.topmetrics|Enable collecting metrics from top admin command||
 |\-\-metrics.overridedescendingindex| Enable descending index name override to replace -1 with _DESC ||
 |--version|Show version and exit|