Remove :nodoc: from Time::Format

[straight-shoota]

Fixes #6206

[bcardiff]

@straight-shoota FYI, the ci complained about formatting issues.

Error: formatting './src/time/format/custom/yaml_date.cr' produced changes

[straight-shoota]

That's weird. My local compiler ( 0.25.0+12 [8e5e4c7f6] (2018-06-18)) didn't complain on bin/crystal tool format src/time/format/custom/yaml_date.cr --check. But running it without --check, it still produced a change, removing an empty line:

--- a/src/time/format/custom/yaml_date.cr
+++ b/src/time/format/custom/yaml_date.cr
@@ -10,7 +10,6 @@ struct Time::Format
   # As an additional note, Ruby's Psych YAML parser also implements a
   # custom time parser, probably for this same reason.
   module YAML_DATE
-
     # Parses a string into a `Time`.
     def self.parse?(string) : Time?
       parser = Parser.new(string)