Renaming section twincat3 to PLC (#5522)

* Rename section TwinCAT3 to PLC

There are more PLC systems than TwinCat-3

Completes EbookFoundation/free-programming-books#5517 review suggestions

* Synchronize TOC

Add missing section present in listing to Table of Contents

courses/free-courses-en.md

@@ -50,6 +50,8 @@
 * [OCaml](#ocaml)
 * [Perl](#perl)
 * [PHP](#php)
+* [PLC - Programable logic controllers](#plc--programable-logic-controllers)
+* [Processing](#processing)
 * [Python](#python)
   * [Django](#django)
   * [Flask](#flask)
@@ -62,7 +64,6 @@
 * [Swift](#swift)
   * [Vapor](#vapor)
 * [Theory](#theory)
-* [TwinCAT 3](#twincat-3)
 * [TypeScript](#typescript)
 * [Verilog / VHDL / SystemVerilog](#verilog--vhdl--systemverilog)
 * [Web Development](#web-development)
@@ -600,6 +601,13 @@
 * [PHP Tutorials](https://www.youtube.com/playlist?list=PL0eyrZgxdwhwBToawjm9faF1ixePexft-) - Dani Krossing
 
 
+### PLC - Programable logic controllers
+
+* [Learning motion control and IO with Beckhoff TwinCAT PLCs](https://www.youtube.com/playlist?list=PLE1CU6EebvTD29gsHo1djsKU7J5Dtp0Bn) - Evan Jensen (YouTube)
+* [Learning PLCs with Structured Text](https://www.youtube.com/playlist?list=PLE1CU6EebvTCJCMIUOSWgMseMaW-2k5zH) - Evan Jensen (YouTube)
+* [PLC programming using TwinCAT 3](https://www.youtube.com/playlist?list=PLimaF0nZKYHz3I3kFP4myaAYjmYk1SowO) - Jakob Sagatowski (YouTube)
+
+
 ### Processing
 
 * [Learning Processing: A Beginner's Guide to Programming Images, Animation, and Interaction](https://www.youtube.com/c/TheCodingTrain/playlists?view=50&sort=dd&shelf_id=10) - The Coding Train
@@ -728,13 +736,6 @@
 * [Intro to Theoretical Computer Science](https://www.udacity.com/course/intro-to-theoretical-computer-science--cs313) (Udacity)
 
 
-### TwinCAT 3
-
-* [Learning motion control and IO with Beckhoff TwinCAT PLCs](https://www.youtube.com/playlist?list=PLE1CU6EebvTD29gsHo1djsKU7J5Dtp0Bn) - Evan Jensen (YouTube)
-* [Learning PLCs with Structured Text](https://www.youtube.com/playlist?list=PLE1CU6EebvTCJCMIUOSWgMseMaW-2k5zH) - Evan Jensen (YouTube)
-* [PLC programming using TwinCAT 3](https://www.youtube.com/playlist?list=PLimaF0nZKYHz3I3kFP4myaAYjmYk1SowO) - Jakob Sagatowski (YouTube)
-
-
 ### TypeScript
 
 * [Introduction to TypeScript](https://www.udemy.com/typescript/) - Daniel Stern (Udemy)