doc: add serial comma in CPP_STYLE_GUIDE.md

PR-URL: #17464
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

Author: Trott

CPP_STYLE_GUIDE.md

@@ -8,7 +8,7 @@
   * [4 spaces of indentation for statement continuations](#4-spaces-of-indentation-for-statement-continuations)
   * [Align function arguments vertically](#align-function-arguments-vertically)
   * [Initialization lists](#initialization-lists)
-  * [CamelCase for methods, functions and classes](#camelcase-for-methods-functions-and-classes)
+  * [CamelCase for methods, functions, and classes](#camelcase-for-methods-functions-and-classes)
   * [snake\_case for local variables and parameters](#snake_case-for-local-variables-and-parameters)
   * [snake\_case\_ for private class fields](#snake_case_-for-private-class-fields)
   * [Space after `template`](#space-after-template)