docs(AIP-132): fix incorrect field for AIP-217 (googleapis#1423)

The field name "unavailable" listed on https://linter.aip.dev/132/response-unknown-fields is incorrect. According to AIP-217, the field should be "unreachable".

docs/rules/0132/response-unknown-fields.md

@@ -21,7 +21,7 @@ comes across any fields other than:
 - The resource.
 - `int32/int64 total_size` ([AIP-132][])
 - `string next_page_token` ([AIP-158][])
-- `repeated string unavailable` ([AIP-217][])
+- `repeated string unreachable` ([AIP-217][])
 
 It only checks field names; it does not validate type correctness or
 repeated-ness.
@@ -71,4 +71,5 @@ top of the file.
 [aip-135]: https://aip.dev/135
 [aip-157]: https://aip.dev/157
 [aip-158]: https://aip.dev/158
+[aip-217]: https://aip.dev/217
 [aip.dev/not-precedent]: https://aip.dev/not-precedent