AVRO-2849: Update spec to clarify valid namespaces (#917)
AVRO-2849: Update spec to clarify valid namespaces (#917)

- Adds the grammar as discussed on the mailing list:
https://lists.apache.org/thread.html/r93c4142922587ceefab78d2695570778b22a51e4bd140fd58c21984b%40%3Cdev.avro.apache.org%3E

- Removes double "the".
apache/avro,apache/avro,apache/avro,apache/avro,apache/avro,apache/avro,apache/avro,apache/avro,apache/avro,apache/avro,apache/avro,apache/avro