standardize the submit button

src/main/resources/templates/security/add-watermark.html

@@ -131,7 +131,7 @@
                             <label for="convertPDFToImage" th:text="#{watermark.selectText.10}"></label>
                         </div>
                         <div class="mb-3 text-left">
-                            <input type="submit" id="submitBtn" th:value="#{watermark.submit}" class="btn btn-primary">
+                            <button type="submit" id="submitBtn" class="btn btn-primary" th:text="#{watermark.submit}"></button>
                         </div>
                     </form>