Switch to namespace = "quoted::path" in docs and tests

To preserve parity with item-level #[namespace = "..."] which is
currently restricted by rustc to a quoted string.
diff --git a/demo/src/main.rs b/demo/src/main.rs
index ee7e093..8f62084 100644
--- a/demo/src/main.rs
+++ b/demo/src/main.rs
@@ -1,4 +1,4 @@
-#[cxx::bridge(namespace = org::example)]
+#[cxx::bridge(namespace = "org::example")]
 mod ffi {
     struct SharedThing {
         z: i32,