Clarify usage ping documentation

Previously the documentation said "Once usage ping is enabled", which
implied usage ping is disabled by default. That was misleading because
usage ping is enabled by default. This replaces "once" with "while" to
avoid an incorrect implication.

doc/development/telemetry/usage_ping.md

@@ -31,7 +31,7 @@ More useful links:
 - The usage data is primarily composed of row counts for different tables in the instance’s database. By comparing these counts month over month (or week over week), we can get a rough sense for how an instance is using the different features within the product. In addition to counts, other facts
     that help us classify and understand GitLab installations are collected.
 - Usage ping is important to GitLab as we use it to calculate our Stage Monthly Active Users (SMAU) which helps us measure the success of our stages and features.
-- Once usage ping is enabled, GitLab will gather data from the other instances and will be able to show usage statistics of your instance to your users.
+- While usage ping is enabled, GitLab will gather data from the other instances and will be able to show usage statistics of your instance to your users.
 
 ### Why should we enable Usage Ping?