Apply Juju SDK docstrings style

Docstring suggestions can be found in the official style guide [1]

[1]: https://juju.is/docs/sdk/styleguide#heading--docstrings

Change-Id: I1e09a980acaae9600dec80e3ea9d07b06ca126fe
diff --git a/src/charm.py b/src/charm.py
index 6e4fb2e..d56b608 100755
--- a/src/charm.py
+++ b/src/charm.py
@@ -28,6 +28,10 @@
 
 @dataclasses.dataclass(frozen=True)
 class StorPoolConfItems:
+    """
+    'Serialize' StorPool configuration items depending on the target file format
+    """
+
     sp_api_http_host: str
     sp_api_http_port: str
     sp_auth_token: str