docs(import_maps): Add trailing slash to ./src example (denoland#8524)

Update docs to reflect behavior. If trailing slash is not provided, `deno run` will throw error `Package address targets must end with "/"`.

docs/linking_to_external_code/import_maps.md

@@ -65,7 +65,7 @@ You may map a different directory: (eg. src)
 
 {
   "imports": {
-    "/": "./src"
+    "/": "./src/"
   }
 }
 ```