chore: minor improvement of English translation

Author: php-coder

Diff for: src/main/resources/ru/mystamps/i18n/Messages.properties

@@ -121,7 +121,7 @@ t_quantity = Quantity
 t_perforated = Perforated
 t_add_catalogs_info = Add information from stamps catalogues
 
-t_add_comment = Add comment
+t_add_comment = Add a comment
 t_comment = Comment
 t_image = Image
 t_add_more_images_hint = Later you will be able to add additional images

Diff for: src/main/webapp/WEB-INF/views/series/add.html

@@ -516,7 +516,7 @@ <h3 th:text="${#strings.capitalize(add_series)}">
 
 						<div class="form-group js-collapse-toggle-header" sec:authorize="hasAuthority('ADD_COMMENTS_TO_SERIES')">
 							<div class="col-sm-offset-3 col-sm-5">
-								<span class="glyphicon glyphicon-chevron-down"></span>&nbsp;<a href="javascript:void(0)" id="add-comment-link" data-toggle="collapse" data-target=".js-comment" th:text="#{t_add_comment}">Add comment</a>
+								<span class="glyphicon glyphicon-chevron-down"></span>&nbsp;<a href="javascript:void(0)" id="add-comment-link" data-toggle="collapse" data-target=".js-comment" th:text="#{t_add_comment}">Add a comment</a>
 							</div>
 						</div>