Fix incorrect library name and path

I think these were a copy and paste error from this file,

https://github.com/CocoaPods/Specs/blob/master/GoogleAds-IMA-iOS-SDK/3.0.beta.5/GoogleAds-IMA-iOS-SDK.podspec
Merge pull request #10955 from themackworth/ad_mob_wrong_library

Fix incorrect library name and path