Update RNTester Podfile.lock (#26976)

Summary:
There were a couple of inconsistencies and outdated checksums in `RNTester`'s `Podfile.lock` - Running `pod install` would leave a clean clone dirty.

This updates Podfile.lock (no manual changes).

## Changelog

[General] [Changed] - Update RNTester Podfile.lock
Pull Request resolved: https://github.com/facebook/react-native/pull/26976

Test Plan:
```
cd RNTester
pod install
```

Verify that `git status` is clean.

**Ensure that RNTesterPods.xcworkspace builds`
1. Run: `open ~/fbsource/xplat/js/react-native-github/RNTester/RNTesterPods.xcworkspace`
2. Press Run
3. **Assert:** The project should compile without error.

Differential Revision: D18102395

Pulled By: RSNara

fbshipit-source-id: a20a9f6b69727fa63d1eaac1a2b1150c6f6400cf

RNTester/Podfile.lock

@@ -308,7 +308,7 @@ DEPENDENCIES:
   - Yoga (from `../ReactCommon/yoga`)
 
 SPEC REPOS:
-  https://github.com/cocoapods/specs.git:
+  https://github.com/CocoaPods/Specs.git:
     - boost-for-react-native
 
 EXTERNAL SOURCES:
@@ -402,4 +402,4 @@ SPEC CHECKSUMS:
 
 PODFILE CHECKSUM: 060903e270072f1e192b064848e6c34528af1c87
 
-COCOAPODS: 1.7.1
+COCOAPODS: 1.8.4