Fix documentation_url in podspec.

Alamofire.podspec

@@ -7,7 +7,7 @@ Pod::Spec.new do |s|
   s.social_media_url = 'http://twitter.com/AlamofireSF'
   s.authors = { 'Alamofire Software Foundation' => 'info@alamofire.org' }
   s.source = { :git => 'https://github.com/Alamofire/Alamofire.git', :tag => s.version }
-  s.documentation_ur = 'https://alamofire.github.io/Alamofire/'
+  s.documentation_url = 'https://alamofire.github.io/Alamofire/'
 
   s.ios.deployment_target = '8.0'
   s.osx.deployment_target = '10.10'