Update eip-998.md (ethereum#2314)

Fix minor typo - creates confusion.

EIPS/eip-998.md

@@ -24,7 +24,7 @@ This specification covers four different kinds of composable tokens:
 
 ## Abstract
 
-1. An ERC988ERC721 top-down composable is an ERC721 token with additional functionality for owning other ERC721 tokens. 
+1. An ERC998ERC721 top-down composable is an ERC721 token with additional functionality for owning other ERC721 tokens. 
 2. An ERC998ERC20 top-down composable is an ERC721 token with additional functionality for owning ERC20 tokens. 
 3. An ERC998ERC721 bottom-up composable is an ERC721 token with additional functionality for being owned by an ERC721 token.
 4. An ERC998ERC20 bottom-up composable is an ERC20 token with additional functionality for being owned by an ERC721 token.