cmd/root.go(rootCmd): Modify description to call it "command", not "s…

…ubcommand".

To me, "subcommand" conjures something like the "s3" in "aws s3", that is, a subcommand of the tool
itself.  Whereas in fact, we mean to refer to external commands that yak will execute on our
behalf.  I think just calling it `<command>` in the docs and help text is clearer.

README.md

@@ -106,7 +106,7 @@ yak [flags] -- <role> <command --with-flags>
   -u, --okta-username string            Your Okta username
   -o, --output-format string            Can be set to either 'json' or 'env'. The format in which to output credential data
       --version                         Print the current version and exit
-      --                                Terminator for -/-- flags. Necessary if you want to pass -/-- flags to subcommands
+      --                                Terminator for -/-- flags. Necessary if you want to pass -/-- flags to commands
 ```
 
 #### Environment Variables

cmd/root.go

@@ -20,19 +20,19 @@ import (
 )
 
 var rootCmd = &cobra.Command{
-	Use:   "yak [flags] [--list-roles | [--] <role> [<subcommand...>]]",
+	Use:   "yak [flags] [--list-roles | [--] <role> [<command...>]]",
 	Short: "A shim to do stuff with AWS credentials using Okta",
 	Long: `A shim to do stuff with AWS credentials using Okta
 
   * With --list-roles, print a list of your available AWS roles.
     Otherwise, yak will attempt to generate AWS keys for <role>.
 
-  * If <subcommand> is set, yak will attempt to execute it with the
+  * If <command> is set, yak will attempt to execute it with the
     AWS keys injected into the environment.  Otherwise, the
     credentials will conveniently be printed stdout.
 
-    Note that if you want to pass -/-- flags to your <subcommand>,
-    you'll need to put a '--' separator before the <role> so yak
+    Note that if you want to pass -/-- flags to your <command>,
+    you'll need to put a '--' separator before the <command> so yak
     knows not to interpret those arguments for itself`,
 	SilenceUsage:  true,
 	SilenceErrors: true,