docs: Fix typo

Simply replacing Extentions with the correct Extensions.
diff --git a/docs/devinfo.html b/docs/devinfo.html
index b495097..38c052a 100644
--- a/docs/devinfo.html
+++ b/docs/devinfo.html
@@ -17,7 +17,7 @@
 <h1>Development Notes</h1>
 
 
-<h2>Adding Extentions</h2>
+<h2>Adding Extensions</h2>
 
 <p>
 To add a new GL extension to Mesa you have to do at least the following.