Fix TOC headings for learning resources

README.md

@@ -31,17 +31,15 @@ pure C stuff is considered.
 * [Graphical User Interface](#graphical-user-interface)
 * [JSON](#json)
 * [Learning, Reference and Tutorials](#learning-reference-and-tutorials)
-  * [Online](#online)
-    * [Reference](#reference)
-    * [Beginner](#beginner)
-    * [Intermediate](#intermediate)
-    * [Advanced](#advanced)
-    * [Self-study courses](#self-study-courses)
-  * [Physical](#physical)
-    * [Reference](#reference-1)
-    * [Beginner](#beginner-1)
-    * [Intermediate](#intermediate-1)
-    * [Advanced](#advanced-1)
+  * [Reference resources online](#reference-resources-online)
+  * [Beginner resources online](#beginner-resources-online)
+  * [Intermediate resources online](#intermediate-resources-online)
+  * [Advanced resources online](#advanced-resources-online)
+  * [Online self-study courses](#online-self-study-courses)
+  * [Reference books](#reference-books)
+  * [Beginner books](#beginner-books)
+  * [Intermediate books](#intermediate-books)
+  * [Advanced books](#advanced-books)
 * [Multimedia](#multimedia)
 * [Networking and Internet](#networking-and-internet)
   * [Web Frameworks](#web-frameworks)