chore: tweak readonly error message (#9816)

* tweak readonly error message

* update tests

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

packages/svelte/src/internal/client/proxy/readonly.js

@@ -42,7 +42,7 @@ export function readonly(value) {
  */
 const readonly_error = (_, prop) => {
 	throw new Error(
-		`Props cannot be mutated, unless used with \`bind:\`. Use \`bind:prop-in-question={..}\` to make \`${prop}\` settable. Fallback values can never be mutated.`
+		`Non-bound props cannot be mutated — use \`bind:<prop>={...}\` to make \`${prop}\` settable. Fallback values can never be mutated.`
 	);
 };
 

packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly/_config.js

@@ -15,5 +15,5 @@ export default test({
 	},
 
 	runtime_error:
-		'Props cannot be mutated, unless used with `bind:`. Use `bind:prop-in-question={..}` to make `count` settable. Fallback values can never be mutated.'
+		'Non-bound props cannot be mutated — use `bind:<prop>={...}` to make `count` settable. Fallback values can never be mutated.'
 });

packages/svelte/tests/runtime-runes/samples/proxy-prop-readonly/_config.js

@@ -15,5 +15,5 @@ export default test({
 	},
 
 	runtime_error:
-		'Props cannot be mutated, unless used with `bind:`. Use `bind:prop-in-question={..}` to make `count` settable. Fallback values can never be mutated.'
+		'Non-bound props cannot be mutated — use `bind:<prop>={...}` to make `count` settable. Fallback values can never be mutated.'
 });