Update command in export_to_third_party.md

Update command to resolve issue #939 by applying change from PR #970 to the 5.11 branch.

Author: Amanda Butler

docs/tools/toolchains/export_to_third_party.md

@@ -54,7 +54,7 @@ has the same flags as if you had compiled with:
 
 For a complete list of supported export toolchains, you can run:
 
-    $ mbed export -h
+    $ mbed export --supported ides
 
 ### Before you export