Update 'proto-out' command to 'proto-out-dir'

The command for exporting proto files and setting the output directory has been updated from 'proto-out' to 'proto-out-dir'. This change has been made both in the README and the grpcurl go file. This makes the command name more descriptive, accurately reflecting its functionality.

README.md

@@ -161,7 +161,7 @@ grpcurl -protoset my-protos.bin list
 grpcurl -import-path ../protos -proto my-stuff.proto list
 
 # Export proto files
-grpcurl -plaintext -proto-out "out_protos" "192.168.100.1:9200" describe Api.Service
+grpcurl -plaintext -proto-out-dir "out_protos" "192.168.100.1:9200" describe Api.Service
 
 
 ```

cmd/grpcurl/grpcurl.go

@@ -151,7 +151,7 @@ var (
 		file if this option is given. When invoking an RPC and this option is
 		given, the method being invoked and its transitive dependencies will be
 		included in the output file.`))
-	protoOut = flags.String("proto-out", "", prettify(`
+	protoOut = flags.String("proto-out-dir", "", prettify(`
 		The name of a directory where the generated .proto files will be written.
 		With the list and describe verbs, the listed or described elements and
 		their transitive dependencies will be written as .proto files in the