Update week-1/3-semantic-html/readme.md

Co-Authored-By: Nate Wright <[email protected]>

Author: Astrid Ronja

week-1/3-semantic-html/readme.md

@@ -2,7 +2,7 @@
 
 There are no semantic tags or attributes being used in the messages for this example. Let's improve it by changing the tags and attributes to semantic HTML code, so that computers can understand what kind of information is being presented.
 Take a look at the picture, which shows an example of a semantic HTML structure.
-![alt text](html.jpg)
+![A visual diagram showing the semantic structure of an HTML page.](html.jpg)
 
 Complete the following changes in the `index.html` file: