fix typo for URL in spec doc
[Specification::DSL] Add a docset url attribute

https://github.com/CocoaPods/CocoaPods/issues/1699
Merge pull request #59 from CocoaPods/docset_url

[Specification::DSL] Add a docset url attribute
Remove extra comma and underscores
Merge pull request #60 from carsonmcdonald/lintermessagefix

Remove extra comma and underscores
Merge branch 'master' into feature/specs-sources