Replace ```-style headers with ''' instead.

Why:
* ```-style headers confuse the syntax-highlighting in my text editor.
* I believe '''-style headers are also recommended by the PEP rST template
  for this level of header, but I haven't confirmed.

Author: davidfstr

pep-0681.rst

@@ -113,7 +113,7 @@ Type checkers supporting this PEP will recognize that the
   c5 = CustomerModel(327, "John Smith", 0)
 
 Decorator function example
-``````````````````````````
+''''''''''''''''''''''''''
 
 .. code-block:: python
 
@@ -136,7 +136,7 @@ Decorator function example
       name: str
 
 Class example
-`````````````
+'''''''''''''
 
 .. code-block:: python
 
@@ -152,7 +152,7 @@ Class example
       name: str
 
 Metaclass example
-`````````````````
+'''''''''''''''''
 
 .. code-block:: python
 
@@ -230,7 +230,7 @@ The following sections provide additional examples showing how these
 parameters are used.
 
 Decorator function example
-``````````````````````````
+''''''''''''''''''''''''''
 
 .. code-block:: python
 
@@ -253,7 +253,7 @@ Decorator function example
       name: str
 
 Class example
-`````````````
+'''''''''''''
 
 .. code-block:: python
 
@@ -284,7 +284,7 @@ Class example
       name: str
 
 Metaclass example
-`````````````````
+'''''''''''''''''
 
 .. code-block:: python
 
@@ -353,7 +353,7 @@ not required:
 
 
 Field descriptor parameters
-```````````````````````````
+'''''''''''''''''''''''''''
 
 Libraries that support dataclass-like semantics and support field
 descriptor classes typically use common parameter names to construct