Update reference/src/glossary.md

Co-Authored-By: Ralf Jung <[email protected]>

Author: gnzlbg

reference/src/glossary.md

@@ -87,7 +87,7 @@ the layout of, e.g., `Option<T>`, from the layout of `T`.
 
 The *call ABI* determines how a type is passed *by value* across a function boundary.
 
-Note: The call ABI is not stable. Currently, it is one of:
+Note: The set of possible call ABIs is not stable. Currently, it consists of:
 
 ```rust,ignore
 enum Abi {