Fix gateway s3 doc to run custom S3 endpoint (#6369)

To pass a custom S3 endpoint in S3 gateway, the user needs
to specify it as an argument after 'minio gateway s3' and not
as '--address' option since this latter specifies the address
to which the gateway should listen.

docs/gateway/s3.md

@@ -33,7 +33,7 @@ As a prerequisite to run Minio S3 gateway on an AWS S3 compatible service, you n
 docker run -p 9000:9000 --name minio-s3 \
  -e "MINIO_ACCESS_KEY=access_key" \
  -e "MINIO_SECRET_KEY=secret_key" \
- minio/minio gateway s3 --address https://s3_compatible_service_endpoint:port
+ minio/minio gateway s3 https://s3_compatible_service_endpoint:port
 ```
 
 ### Using Binary
@@ -41,7 +41,7 @@ docker run -p 9000:9000 --name minio-s3 \
 ```
 export MINIO_ACCESS_KEY=access_key
 export MINIO_SECRET_KEY=secret_key
-minio gateway s3 --address https://s3_compatible_service_endpoint:port
+minio gateway s3 https://s3_compatible_service_endpoint:port
 ```
 
 ## Minio Caching