fix(README): use the correct RewardedAd property name

[enricoangelon]

Description

The property named 'adLoadCallback' of RewardedAd.load() shown in the example is outdated, It has been replaced by 'rewardedAdLoadCallback'.

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[malandr2]

@enricoangelon thanks for the fix. I can merge as soon as the missing CLA test is resolved

[malandr2]

Closing due to inactivity - will address in a separate PR