fix: use consistent words and add clarification for semi modular extension (#163)

Author: fangting-alchemy

standard/ERCs/erc-6900.md

@@ -572,7 +572,7 @@ The `executeFromModuleExternal` function MUST allow modules to call external add
 
 #### Semi-modular account
 
-Account implementers may opt to design a semi-modular account, where certain features, such as default validation, are integrated into the core account. This approach should ensure compatibility with fully modular accounts, maintaining interoperability across different implementations.
+Account implementers MAY choose to design a semi-modular account, where certain features, such as default validation, are integrated into the core account. This approach SHOULD ensure compatibility with fully modular accounts, as defined in this proposal, to maintain interoperability across different implementations.
 
 ## Rationale