Docs about periods in log and error messages

The recent documentation on log messages and error messages did not
clarify what the recommendations were about periods at the end of
the sentences.

This change clarifies that.

Signed-off-by: silvamatteus <silvamatteus@lsd.ufcg.edu.br>

CONTRIBUTING.md

@@ -205,6 +205,8 @@ Log messages should make use of logging fields to convey additional information,
 using string formatting which increases the cardinality of messages for log watchers to
 look for and hinders aggregation.
 
+Log messages and error messages should not end with periods.
+
 ## Mocks
 
 Unit tests should avoid mock tests as much as possible. When necessary we should inject mocked