[ios, macos] Correct podspecs, readmes (#7730)

* [macos] RC 1 errata

Synchronized bundled README installation instructions with those in INSTALL.md. Bumped screenshot up a bit. Corrected link to published documentation.

* [ios, macos] Corrected podspec URLs

Fixed podspec linter errors. The podspecs were pointing to unreachable (or soon to be unreachable) screenshot URLs. The -symbols podspecs’ names additionally needed to differ from the normal podspecs’ names.

platform/ios/Mapbox-iOS-SDK.podspec

@@ -1,14 +1,16 @@
 Pod::Spec.new do |m|
 
+  version = '3.4.0-beta.7'
+
   m.name    = 'Mapbox-iOS-SDK'
-  m.version = '3.4.0-beta.7'
+  m.version = version
 
   m.summary           = 'Open source vector map solution for iOS with full styling capabilities.'
   m.description       = 'Open source, OpenGL-based vector map solution for iOS with full styling capabilities and Cocoa Touch APIs.'
   m.homepage          = 'https://www.mapbox.com/ios-sdk/'
   m.license           = { :type => 'BSD', :file => 'LICENSE.md' }
   m.author            = { 'Mapbox' => 'mobile@mapbox.com' }
-  m.screenshot        = 'https://raw.githubusercontent.com/mapbox/mapbox-gl-native/master/platform/ios/screenshot.png'
+  m.screenshot        = "https://www.mapbox.com/ios-sdk/api/#{version}/img/screenshot.png"
   m.social_media_url  = 'https://twitter.com/mapbox'
   m.documentation_url = 'https://www.mapbox.com/ios-sdk/api/'