Rename yoga.podspec to Yoga.podspec (#26360)

Summary:
When doing RN init from scratch for 0.61, I noticed pod install was failing. This was because "Yoga.podspec" was mis-spelled to "yoga.podspec" in the yoga ReactCommon folder. Thus, `pod install` was failing

```
[!] No podspec found for `Yoga` in `../node_modules/react-native/ReactCommon/yoga
```
## Changelog

[iOS] [Fixed] - Fix Yoga.podspec case sensitivity
Pull Request resolved: #26360

Test Plan: `pod install` now runs successfully with a clean build of master.

Differential Revision: D17314027

Pulled By: cpojer

fbshipit-source-id: 8db2ac4dd9295afcc0e074c842dcda71b7b3b668