changing <b> to <strong> in HTML tutorial 3, to keep the tutorial consistent

Author: matyikriszta

html/lesson3/tutorial.md

@@ -232,7 +232,7 @@ Add a class `sidebar` to the div we just defined. This is so that we can change
 
 ```html
 <div class="sidebar">
-  <b>Me on the internet</b>
+  <strong>Me on the internet</strong>
   ...
 ```