further bug fixes when using context path (#1475)

Co-authored-by: Anthony Stirling <77850077+Frooodle@users.noreply.github.com>

src/main/resources/templates/convert/book-to-pdf.html

@@ -14,7 +14,7 @@
           <div class="row justify-content-center">
             <div class="col-md-6">
               <h2 th:text="#{BookToPDF.header}"></h2>
-              <form method="post" enctype="multipart/form-data" th:action="@{api/v1/convert/book/pdf}">
+              <form method="post" enctype="multipart/form-data" th:action="@{'/api/v1/convert/book/pdf'}">
                 <div th:replace="~{fragments/common :: fileSelector(name='fileInput', multiple=false)}"></div>
                 <button type="submit" id="submitBtn" class="btn btn-primary" th:text="#{BookToPDF.submit}"></button>
               </form>