Add proper backtick quotes around literals

Author: chreke

README.rst

@@ -180,11 +180,11 @@ Pass custom defined media type deserializers dictionary with supported mimetypes
 Formats
 *******
 
-OpenAPI defines a `format` keyword that hints at how a value should be interpreted, e.g. a `string` with the type `date` should conform to the RFC 3339 date format.
+OpenAPI defines a ``format`` keyword that hints at how a value should be interpreted, e.g. a ``string`` with the type ``date`` should conform to the RFC 3339 date format.
 
 Openapi-core comes with a set of built-in formatters, but it's also possible to add support for custom formatters for `RequestValidator` and `ResponseValidator`.
 
-Here's how could add support for a `usdate` format that handles dates of the form MM/DD/YYYY:
+Here's how could add support for a ``usdate`` format that handles dates of the form MM/DD/YYYY:
 
 .. code-block:: python