Change used table elements (header -> data cell)

Signed-off-by: Pekka Helenius <fincer89@hotmail.com>

bookstore/src/main/resources/templates/booklist.html

@@ -150,13 +150,13 @@ Idea of the following syntax used in this and other HTML document:
 					button.book.add
 					</a>
 				</td>
-				<th></th>
-				<th></th>
-				<th></th>
-				<th></th>
-				<th></th>
-				<th></th>
-				<th></th>
+				<td></td>
+				<td></td>
+				<td></td>
+				<td></td>
+				<td></td>
+				<td></td>
+				<td></td>
 				<td>
 					<a class="btn btn-dark" th:href="@{__${restpage}__} + '/' + @{__${listpage}__}"
 					th:text="${#messages.msgOrNull('button.page.list.json')} ?: 'button.page.list.json'">
@@ -172,14 +172,14 @@ Idea of the following syntax used in this and other HTML document:
 					button.page.apiref
 					</a>
 				</td>
-				<th></th>
-				<th></th>
-				<th></th>
-				<th></th>
-				<th></th>
-				<th></th>
-				<th></th>
-				<th></th>
+				<td></td>
+				<td></td>
+				<td></td>
+				<td></td>
+				<td></td>
+				<td></td>
+				<td></td>
+				<td></td>
 			</tr>
 
 		</table>