update on_demand_inputs invalidation

book/src/common_patterns/on_demand_inputs.md

@@ -38,12 +38,14 @@ struct MyDatabase { ... }
 impl FileWatcher for MyDatabase {
     fn watch(&self, path: &Path) { ... }
     fn did_change_file(&mut self, path: &Path) {
-        self.query_mut(ReadQuery).invalidate(path);
+        ReadQuery.in_db_mut(self).invalidate(path);
     }
 }
 ```
 
-* We declare the query as a derived query (which is the default).
-* In the query implementation, we don't call any other query and just directly read file from disk.
-* Because the query doesn't read any inputs, it will be assigned a `HIGH` durability by default, which we override with `report_synthetic_read`.
-* The result of the query is cached, and we must call `invalidate` to clear this cache.
+- We declare the query as a derived query (which is the default).
+- In the query implementation, we don't call any other query and just directly read file from disk.
+- Because the query doesn't read any inputs, it will be assigned a `HIGH` durability by default, which we override with `report_synthetic_read`.
+- The result of the query is cached, and we must call `invalidate` to clear this cache.
+
+A complete, runnable file-watching example can be found in [this git repo](https://github.com/ChristopherBiscardi/salsa-file-watch-example/blob/f968dc8ea13a90373f91d962f173de3fe6ae24cd/main.rs) along with [a write-up](https://www.christopherbiscardi.com/on-demand-lazy-inputs-for-incremental-computation-in-salsa-with-file-watching-powered-by-notify-in-rust) that explains more about the code and what it is doing.