Add RNTester Pods to gitignore (#26265)

Summary:
Pods directory showed up as untracked files after running `pod install`
in the `RNTester` directory. With this small change to `.gitignore`, it
will no longer show up.

## Changelog

Small repo-level change, no changelog needed (?)
Pull Request resolved: #26265

Test Plan:
Run `pod install` in `/RNTester`, verify that "Pods" no longer show up
as untracked files in `git status`.

Differential Revision: D17259103

Pulled By: cpojer

fbshipit-source-id: 72a3774bcb48c88c2c6629bf452ddb0cbc32f581

.gitignore

@@ -80,5 +80,4 @@ RNTester/build
 # CocoaPods
 /template/ios/Pods/
 /template/ios/Podfile.lock
-RNTester/Pods/Local Podspecs
-RNTester/Pods/Target Support Files
+/RNTester/Pods/