s/grpc_credentials/grpc_credential/ in api (#3595)

Fixes #3584

Description:
Risk Level: Low
Testing: existing tests / CI
Docs Changes:
Release Notes:

Signed-off-by: Lizan Zhou <zlizan@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ e931af6a84013a934a40f817d941e6b745cbca26

docs/BUILD

@@ -47,7 +47,7 @@ proto_library(
         "//envoy/config/filter/network/rate_limit/v2:rate_limit",
         "//envoy/config/filter/network/redis_proxy/v2:redis_proxy",
         "//envoy/config/filter/network/tcp_proxy/v2:tcp_proxy",
-        "//envoy/config/grpc_credentials/v2alpha:file_based_metadata",
+        "//envoy/config/grpc_credential/v2alpha:file_based_metadata",
         "//envoy/config/health_checker/redis/v2:redis",
         "//envoy/config/metrics/v2:metrics_service",
         "//envoy/config/metrics/v2:stats",

envoy/config/grpc_credential/v2alpha/file_based_metadata.proto

@@ -3,7 +3,7 @@ syntax = "proto3";
 // [#protodoc-title: Grpc Credentials File Based Metadata]
 // Configuration for File Based Metadata Grpc Credentials Plugin
 
-package envoy.config.grpc_credentials.v2alpha;
+package envoy.config.grpc_credential.v2alpha;
 option go_package = "v2alpha";
 
 import "envoy/api/v2/core/base.proto";