Fix the ERC721 link in the ERC1155 doc (#2694)

docs/modules/ROOT/pages/erc1155.adoc

@@ -2,7 +2,7 @@
 
 ERC1155 is a novel token standard that aims to take the best from previous standards to create a xref:tokens.adoc#different-kinds-of-tokens[*fungibility-agnostic*] and *gas-efficient* xref:tokens.adoc#but_first_coffee_a_primer_on_token_contracts[token contract].
 
-TIP: ERC1155 draws ideas from all of xref:erc20.adoc[ERC20], xref:721.adoc[ERC721], and xref:erc777.adoc[ERC777]. If you're unfamiliar with those standards, head to their guides before moving on.
+TIP: ERC1155 draws ideas from all of xref:erc20.adoc[ERC20], xref:erc721.adoc[ERC721], and xref:erc777.adoc[ERC777]. If you're unfamiliar with those standards, head to their guides before moving on.
 
 [[multi-token-standard]]
 == Multi Token Standard