PEP 673: Fix a couple of typos (#2419)

* Name this protocol to match usage

It would have been possible here to change the bound of the TypeVar
on line 495 to 'Shape', however I've opted to change the names of
the definitions so that they match the later (similar) definitions
of this protocol.

* Fix example generic class name

I'm guessing this was just a copy/pasta error or similar; there is
no generic class named 'Foo' which is in scope here, however there
is a 'Bar' that has just been defined and isn't yet used.

pep-0673.rst

@@ -479,7 +479,7 @@ Use in Protocols
 
     from typing import Protocol, Self
 
-    class Shape(Protocol):
+    class ShapeProtocol(Protocol):
         scale: float
 
         def set_scale(self, scale: float) -> Self:
@@ -494,7 +494,7 @@ is treated equivalently to:
 
     SelfShape = TypeVar("SelfShape", bound="ShapeProtocol")
 
-    class Shape(Protocol):
+    class ShapeProtocol(Protocol):
         scale: float
 
         def set_scale(self: SelfShape, scale: float) -> SelfShape:
@@ -658,7 +658,7 @@ The following uses of ``Self`` are rejected.
     class Bar(Generic[T]):
         def bar(self) -> T: ...
 
-    class Baz(Foo[Self]): ...  # Rejected
+    class Baz(Bar[Self]): ...  # Rejected
 
 We reject type aliases containing ``Self``. Supporting ``Self``
 outside class definitions can require a lot of special-handling in