[iOS] fix `pod install` by rename rn-splash-screen.podspec to RCTSplashScreen.podspec

[Ranatchai]

pod install doesn't work if ".podspec" file doesn't have the same name of the project

[davidstoker]

For anyone else with this issue, you can specify the complete path to the podspec to have it work correctly even with different naming:

pod 'RCTSplashScreen', :path => '../node_modules/rn-splash-screen/rn-splash-screen.podspec'

[lanjinglingxx]

The name of the given podspec RCTSplashScreen doesn't match the expected one rn-splash-screen

[Psiiirus]

pls merge

[dazjean]

pls merge