Modify a docstring to avoid exceeding line length limit

A long line in this docstring was causing the flake8 pre-commit check to
fail, so I edited the comment to be compliant. I shortened the lines to
about 80 characters for consistency with other comments, even though
that's more of a change than is strictly required to pass flake8. With
this change, we can start requiring pre-commit to pass for all merges.

I also changed a note to use Restructured Text syntax, for compatibility
with other docstrings in the project.

pytest_localserver/https.py

@@ -126,9 +126,12 @@ def __init__(self, host="localhost", port=0, key=DEFAULT_KEY, cert=DEFAULT_CERTI
     def certificate(self):
         """
         Returns the path to the server's SSL/TLS certificate file.
-        Clients can use this path to access and verify the server's identity by incorporating the certificate.
+        Clients can use this path to access and verify the server's identity by
+        incorporating the certificate.
 
-        Note: Do not rely on having a stable filesystem path for the returned certificate path across different versions or test runs.
+        .. note::
+            Do not rely on having a stable filesystem path for the returned
+            certificate path across different versions or test runs.
         """
         return self._cert