cmd: fix a comment typo

Author: creachadair

cmd/go-cache-plugin/help.go

@@ -18,7 +18,7 @@ Parameters can be passed either as flags or via environment variables.
 See also "help environment".
 
 The plugin requires credentials to access S3. If you are running in AWS, it can
-get credentials from the instnce metadata service; otherwise you will need to
+get credentials from the instance metadata service; otherwise you will need to
 plumb AWS environment variables (AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY) or
 set up a configuration file.