Update README.md

I have updated the android manifest CampaignTrackingService snippet, removing the forward slash "/". 
from:
```
"<receiver android:name="com.google.analytics.tracking.android.CampaignTrackingReceiver" android:exported="true" />"
```
to:
```
"<receiver android:name="com.google.analytics.tracking.android.CampaignTrackingReceiver" android:exported="true" >"
```
This prevents XML exception on build.