Cookbook.nancy.md: Add an example of $filename

Author: Alistair Turnbull

Cookbook.md

@@ -237,3 +237,9 @@ Sometimes we would like to process input files using a command other than `nancy
 ```
 $run(date,-d,@$run(stat,-c%Y,$realpath),+%F)
 ```
+
+We might want to process a different file. For example, [`timeplate.in.html`](#website-example) could display the modification date of `main.in.html` using `$filename` like this:
+
+```
+$run(date,-d,@$run(stat,-c%Y,$filename(main.in.html)),+%F)
+```

Cookbook.nancy.md

@@ -134,3 +134,9 @@ Sometimes we would like to process input files using a command other than `nancy
 ```
 \$run(date,-d,@$run(stat,-c%Y,$realpath),+%F)
 ```
+
+We might want to process a different file. For example, [`timeplate.in.html`](#website-example) could display the modification date of `main.in.html` using `\$filename` like this:
+
+```
+\$run(date,-d,@$run(stat,-c%Y,$filename(main.in.html)),+%F)
+```