Use absolute URLs for README images

There appears to be an issue with the README image rendering on the GitHub
actions storefront that results in broken images. This commit changes the image
references to absolute URLs to try and work around the problem.

README.md

@@ -54,10 +54,10 @@ The action supports the Github Code Scanning integration, which will push slithe
 ## Code Scanning preview
 
 ### Findings Summary
-<img src="./images/summary.png" alt="Summary" width="500"/>
+<img src="https://raw.githubusercontent.com/crytic/slither-action/68ad2434d613601b79da77aeb6b3bb04024d3d10/images/summary.png" alt="Summary" width="500"/>
 
 ### Findings Details
-<img src="./images/details.png" alt="Summary" width="500"/>
+<img src="https://raw.githubusercontent.com/crytic/slither-action/68ad2434d613601b79da77aeb6b3bb04024d3d10/images/details.png" alt="Summary" width="500"/>
 
 ## How to use