Updated index.md

Without -n option to echo. Without the -n the file would contain a newline at the end.
https://scottlinux.com/2012/09/01/encode-or-decode-base64-from-the-command-line/ as reference

docs/user-guide/secrets/index.md

@@ -47,8 +47,8 @@ username and password that the pods should use is in the files
 
 ```shell
 # Create files needed for rest of example.
-$ echo "admin" > ./username.txt
-$ echo "1f2d1e2e67df" > ./password.txt
+$ echo -n "admin" > ./username.txt
+$ echo -n "1f2d1e2e67df" > ./password.txt
 ```
 
 The `kubectl create secret` command