bpo-45463: Clarify that global statements allows multiple names (GH-28851)

bpo-45463: Clarify that global statements allows multiple names (GH-28851)

The global statement allows specifying a list of identifiers
(https://docs.python.org/3/reference/simple_stmts.html#the-global-statement).

The "Execution model" chapter described the global statement as if it
only allowed one single name. Pluralize "name" in the appropriate places.