Add reflection service to .spi.yml (grpc#1742)

Motivation:

No docs are generated on Swift Package Index for the reflection service
as it isn't included in the configuration.

Modifications:

Add reflection service to .spi.yml

Result:

Docs generate for the reflection service

Author: glbrntt

.spi.yml

@@ -1,4 +1,4 @@
 version: 1
 builder:
   configs:
-    - documentation_targets: [GRPC, protoc-gen-grpc-swift]
+    - documentation_targets: [GRPC, GRPCReflectionService, protoc-gen-grpc-swift]