Revert "Improve CocoaPods instructions (pointfreeco#158)"

This reverts commit 7c350f6.

SnapshotTesting appears to not be compatible with this style of Podfile:

CocoaPods/CocoaPods#8318

Let's go back to the old description for now.

README.md

@@ -143,13 +143,11 @@ github "pointfreeco/swift-snapshot-testing" ~> 1.1
 
 ### CocoaPods
 
-If your project uses [CocoaPods](https://cocoapods.org), add the pod to applicable _test_ targets in your `Podfile`:
+If your project uses [CocoaPods](https://cocoapods.org), add the pod to any applicable test targets in your `Podfile`:
 
 ```ruby
-target 'MyApp' do
-  target 'MyAppTests' do
-    pod 'SnapshotTesting', '~> 1.1'
-  end
+target 'MyAppTests' do
+  pod 'SnapshotTesting', '~> 1.1'
 end
 ```