Update SECURITY.md

Author: Gazler

SECURITY.md

@@ -5,12 +5,12 @@
 Phoenix applies bug fixes only to the latest minor branch. Security patches are
 available for the last 4 minor branches:
 
-Elixir version | Support
-:------------- | :-----------------------------
-1.7            | Bug fixes and security patches
-1.6            | Security patches only
-1.5            | Security patches only
-1.4            | Security patches only
+Phoenix version | Support
+:-------------- | :-----------------------------
+1.7             | Bug fixes and security patches
+1.6             | Security patches only
+1.5             | Security patches only
+1.4             | Security patches only
 
 ## Announcements