[ruby/logger] Fix the Logger::Formatter documentation

ruby/logger@db554fbda7

Author: liijunwei

lib/logger.rb

@@ -107,7 +107,7 @@
 #
 # - \Severity (one letter).
 # - Timestamp.
-# - Timezone.
+# - Process id.
 # - \Severity (word).
 # - Program name.
 # - Message.