[ERC] Fixes Compatibility and Versioning

Added explicit reference to ethereum#831

EIPS/eip-681.md

@@ -76,7 +76,7 @@ Similarly `gasLimit` and `gasPrice` are suggested user-editable values for *gas
 The proposed format is chosen to resemble `bitcoin:` URLs as closely as possible, as both users and application programmers are already familiar with that format. In particular, this motivated the omission of the unit, which is often used in Ethereum ecosystem. Handling different orders of magnitude is delegated to the application, just like in the case of `bitcoin:`, but lacking access to the block chain, the application can take a hint from the exponent in the URL. Additional parameters may be added, if popular use cases requiring them emerge in practice.
 
 ## Compatibility and Versioning
-Future upgrades that are partially or fully incompatible with this proposal should introduce a version prefix to `target_address` that is separated by a dash (`-`) character from whatever follows it. Exact specifications lie outside the scope of this document.
+Future upgrades that are partially or fully incompatible with this proposal must use a prefix other than `pay-` that is separated by a dash (`-`) character from whatever follows it, as specified by ERC #831.
 
 ## Copyright