es_datum: fix grammar in doc

Author: mikpe

src/es_datum.erl

@@ -1,6 +1,6 @@
 %%% -*- erlang-indent-level: 2 -*-
 %%%
-%%%   Copyright 2014-2022 Mikael Pettersson
+%%%   Copyright 2014-2025 Mikael Pettersson
 %%%
 %%%   Licensed under the Apache License, Version 2.0 (the "License");
 %%%   you may not use this file except in compliance with the License.
@@ -86,7 +86,7 @@
 %%%
 %%% procedure           Fun/N
 %%%
-%%% An Scheme procedure becomes an Erlang function of the same arity.
+%%% A Scheme procedure becomes an Erlang function of the same arity.
 %%% Variable-arity procedures are not supported.
 %%%
 %%% exception          {Class, Reason, Stack}