[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/macos/Mapbox-macOS-SDK.podspec

@@ -1,24 +1,26 @@
 Pod::Spec.new do |m|
 
+  version = '0.3.0-rc.1'
+
   m.name    = 'Mapbox-macOS-SDK'
-  m.version = '0.3.0-rc.1'
+  m.version = version
 
   m.summary           = 'Open-source, interactive, fully customizable vector maps.'
   m.description       = 'Interactive, fully customizable vector maps with tight platform integration and high-performance OpenGL rendering.'
   m.homepage          = 'https://mapbox.github.io/mapbox-gl-native/macos/'
   m.license           = { :type => 'BSD', :file => 'LICENSE.md' }
   m.author            = { 'Mapbox' => 'mobile@mapbox.com' }
-  m.screenshot        = "https://raw.githubusercontent.com/mapbox/mapbox-gl-native/macos-#{m.version.to_s}/platform/macos/docs/img/screenshot.jpg"
+  m.screenshot        = "https://mapbox.github.io/mapbox-gl-native/macos/#{version}/img/screenshot.jpg"
   m.social_media_url  = 'https://twitter.com/mapbox'
   m.documentation_url = 'https://mapbox.github.io/mapbox-gl-native/macos/'
 
   m.source = {
-    :http => "https://github.com/mapbox/mapbox-gl-native/releases/download/macos-v#{m.version.to_s}/mapbox-macos-sdk-#{m.version.to_s}.zip",
+    :http => "https://github.com/mapbox/mapbox-gl-native/releases/download/macos-v#{version}/mapbox-macos-sdk-#{m.version.to_s}.zip",
     :flatten => true
   }
 
-  m.platform              = :macos
-  m.macos.deployment_target = '10.10'
+  m.platform              = :osx
+  m.osx.deployment_target = '10.10'
 
   m.requires_arc = true