sketch-app-shell: fetch "cancel" not "/cancel"

Fixes https://github.com/boldsoftware/sketch/issues/22
diff --git a/webui/src/web-components/sketch-app-shell.ts b/webui/src/web-components/sketch-app-shell.ts
index 21bbb9e..a540144 100644
--- a/webui/src/web-components/sketch-app-shell.ts
+++ b/webui/src/web-components/sketch-app-shell.ts
@@ -1112,7 +1112,7 @@
     ) as HTMLButtonElement;
     stopButton?.addEventListener("click", async () => {
       try {
-        const response = await fetch("/cancel", {
+        const response = await fetch("cancel", {
           method: "POST",
           headers: {
             "Content-Type": "application/json",