modify sample_n_per_3_secs notes (#4185)

Negative or zero means off, by default

apm-sniffer/config/agent.config

@@ -21,7 +21,7 @@
 agent.service_name=${SW_AGENT_NAME:Your_ApplicationName}
 
 # The number of sampled traces per 3 seconds
-# Negative number means sample traces as many as possible, most likely 100%
+# Negative or zero means off, by default
 # agent.sample_n_per_3_secs=${SW_AGENT_SAMPLE:-1}
 
 # Authentication active is based on backend setting, see application.yml for more details.