docs(learn): fix subject-verb agreement in Docker env var tip

Review feedback on #643: "makes `-e` flags the natural way" mismatched
plural subject with singular complement. Gerund "using" fixes it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: Ethan-Arrowood

learn/getting-started/install-and-connect-harper.mdx

@@ -111,7 +111,7 @@ The image is configured to automatically run the `harper` command upon startup.
 The Harper installation process is normally an interactive prompt; however, it also supports overrides using environment variables or CLI arguments. Since the image contains preset environment variables and additional environment variables `DEFAULTS_MODE` and `REPLICATION_HOSTNAME` are included in the `docker run` command, Harper will complete its installation step completely non-interactively.
 
 :::tip
-This is not limited to installation: _any_ Harper configuration option can be set with an environment variable by mapping its YAML key to `SCREAMING_SNAKE_CASE` (`http.port` becomes `HTTP_PORT`, `replication.hostname` becomes `REPLICATION_HOSTNAME`), which makes `-e` flags the natural way to configure a containerized instance. See [Environment Variables](/reference/v5/configuration/overview#2-environment-variables) in the configuration reference for the full naming convention.
+This is not limited to installation: _any_ Harper configuration option can be set with an environment variable by mapping its YAML key to `SCREAMING_SNAKE_CASE` (`http.port` becomes `HTTP_PORT`, `replication.hostname` becomes `REPLICATION_HOSTNAME`), which makes using `-e` flags the natural way to configure a containerized instance. See [Environment Variables](/reference/v5/configuration/overview#2-environment-variables) in the configuration reference for the full naming convention.
 :::
 
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