Documentation for FCM batch request limit is incorrect

[178inaba]

The FCM batch request limit has been changed from 100 to 500 with the following pull request.
#297

But the documentation doesn't seem to be fixed.

firebase-admin-go/messaging/messaging_batch.go

Line 92 in e088f55

// The messages array may contain up to 100 messages. SendAll employs batching to send the entire

[google-oss-bot]

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[178inaba]

Since this issue is a document, I didn't write it because I didn't think it needed environment or reproduce.