Remove the example using :branch in the spec source

Using `:branch` in a podspec is a terrible idea since it’s a moving target. It’s also not SemVer compliant at all.
Use spec.version.to_s as example for :tag in source

Having a tag matching the version number is a good practice, most pods do it already. It’s also more DRY.
Use :tag instead of :commit in the submodules example

Podspec authors should be encouraged to use tags. Specifying the source with :branch or :commit should be reserved for podfiles.

Also using a pod actually having submodules instead of AFNetworking to reduce potential confusion.
Merge pull request #155 from CocoaPods/better-podspec-source-examples

Better podspec source examples