docs: fix grpc protocol configuration name (#13830)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
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#### Description

`grpc/protobuf` is not a valid configuration option for the otelconf
protocol configuration, `grpc` should be used instead. See:
https://github.com/open-telemetry/opentelemetry-go-contrib/blob/main/otelconf/v0.3.0/config.go#L30C26-L30C30

Co-authored-by: Damien Mathieu <[email protected]>

Author: rogercoll

docs/observability.md

@@ -125,14 +125,14 @@ service:
             interval: 5000
             exporter:
               otlp:
-                protocol: grpc/protobuf
+                protocol: grpc
                 endpoint: https://backend:4317
     traces:
       processors:
         - batch:
             exporter:
               otlp:
-                protocol: grpc/protobuf
+                protocol: grpc
                 endpoint: https://backend2:4317
 ```
 
@@ -153,7 +153,7 @@ service:
         batch:
           exporter:
             otlp:
-              protocol: grpc/protobuf
+              protocol: grpc
               endpoint: ${MY_POD_IP}:4317
 ```