Repository: openconfig/gnmic (language: go)

Issue:
Wrong path in config option
The usage output says `gnmic.yaml` instead of `.gnmic.yaml`.

```
--config string            config file (default is $HOME/gnmic.yaml)
```

Also the [documentation page](https://gnmic.openconfig.net/user_guide/configuration_file/) does not mention anything about the default configuration file name.


Discussion / hints:
you are right, the usage output is outdated.
the documentation page that explains the config files name and location is [here](https://gnmic.openconfig.net/user_guide/configuration_intro/#file-configuration)

Fix patch:
diff --git a/app/app.go b/app/app.go
index 989f5a0c..c383850b 100644
--- a/app/app.go
+++ b/app/app.go
@@ -158,7 +158,7 @@ func (a *App) Context() context.Context {
 func (a *App) InitGlobalFlags() {
 	a.RootCmd.ResetFlags()
 
-	a.RootCmd.PersistentFlags().StringVar(&a.Config.CfgFile, "config", "", "config file (default is $HOME/gnmic.yaml)")
+	a.RootCmd.PersistentFlags().StringVar(&a.Config.CfgFile, "config", "", "main config file")
 	a.RootCmd.PersistentFlags().StringSliceVarP(&a.Config.GlobalFlags.Address, "address", "a", []string{}, "comma separated gnmi targets addresses")
 	a.RootCmd.PersistentFlags().StringVarP(&a.Config.GlobalFlags.Username, "username", "u", "", "username")
 	a.RootCmd.PersistentFlags().StringVarP(&a.Config.GlobalFlags.Password, "password", "p", "", "password")


Test patch: