Update extensions/README.md

Co-authored-by: Thomas Fossati <[email protected]>

extensions/README.md

@@ -22,7 +22,7 @@ types. This is done via three distinct extension mechanisms:
      containing structure by defining an appropriate validation method for it.
 
    This corresponds to the map-extension sockets in the spec.
-2. Some fields may have values one of a set of pre-determined types (e.g. an
+2. Some fields may have values chosen from a set of pre-determined types (e.g., an
    instance ID may be either a UUID or UEID). These (mostly) correspond to
    type-choice sockets in the CoRIM spec. The set of allowed types for a field
    may be extended by registering a factory function for a new type, using the