README: move screenshot

The screenshot is not used in the tests, so doesn't belong in the `tests` directory.
Putting it in the `doc` directory seems more appropriate.

Author: jrfnl

README.md

@@ -42,7 +42,7 @@ This PHAR file is published as an asset for each release and can be found on the
 
 ## Example output
 
-![Example use of tool with error](/tests/examples/example-images/use-error.png?raw=true "Example use of tool with error")
+![Example use of tool with error](/doc/use-error.png?raw=true "Example use of tool with error")
 
 
 ## History