Asset Tag: Correct parameter name

Doc says to use parameter `max_items` which does not seem to work. Corrected to `max_files` - which works as expected - as documented here: https://docs.statamic.com/fieldtypes/assets

Author: PhilJ

content/collections/tags/assets.md

@@ -154,7 +154,7 @@ bacon_images:
 
 ### Single Assets {#single-assets}
 
-If you have an asset field with `max_items: 1` the data will be saved as a `string`. As one cannot iterate over a string, the tag will adjust accordingly without complaining.
+If you have an asset field with `max_files: 1` the data will be saved as a `string`. As one cannot iterate over a string, the tag will adjust accordingly without complaining.
 
 ``` .language-yaml
 hero_image: /img/negasonic-teenage-warhead.jpg