Update index.md

Simply add the values for the constants.
diff --git a/doc/index.md b/doc/index.md
index 724c148..f8bcbeb 100644
--- a/doc/index.md
+++ b/doc/index.md
@@ -253,10 +253,10 @@
 
 ### Constants
 
-- `FileTransferError.FILE_NOT_FOUND_ERR`
-- `FileTransferError.INVALID_URL_ERR`
-- `FileTransferError.CONNECTION_ERR`
-- `FileTransferError.ABORT_ERR`
+- 1 = `FileTransferError.FILE_NOT_FOUND_ERR`
+- 2 = `FileTransferError.INVALID_URL_ERR`
+- 3 = `FileTransferError.CONNECTION_ERR`
+- 4 = `FileTransferError.ABORT_ERR`
 
 ## Backwards Compatibility Notes