PEP 591: some minor cleanups (#994)

- Fix some typos
 - Clarify that @final can't be used on non-method functions

pep-0591.rst

@@ -57,7 +57,7 @@ situations:
   subclasses. (``@property`` can make an attribute read-only but
   does not prevent overriding)
 * Allowing a name to be used in situations where ordinarily a literal
-  is expected (for example as a file name for ``NamedTuple``, a tuple
+  is expected (for example as a field name for ``NamedTuple``, a tuple
   of types passed to ``isinstance``, or an argument to a function
   with arguments of ``Literal`` type [#PEP-586]_).
 
@@ -115,6 +115,8 @@ implementation (or on the first overload, for stubs)::
        def method(self, x=None):
            ...
 
+It is an error to use ``@final`` on a non-method function.
+
 The ``Final`` annotation
 ------------------------
 
@@ -175,8 +177,6 @@ names in type-checked code::
    for x in [1, 2, 3]:
       FOO: Final = x  # Error: Cannot use Final inside a loop
 
-error: Cannot use Final inside a loop
-
 Additionally, a type checker should prevent final attributes from
 being overridden in a subclass::