Opta, ModBus RTU: Fix headings (#882)

Author: marqdevx

content/hardware/05.pro-solutions/solutions-and-kits/opta/tutorials/getting-started-with-modbus-rtu/content.md

@@ -34,7 +34,7 @@ The Modbus RTU protocol is one of the protocols available within Opta™. In thi
 
 ### Required Hardware and Software
 
-### Hardware Requirements
+#### Hardware Requirements
 
 - Opta™ PLC with RS-485 support (x2)
 - 12VDC/1A DIN rail power supply (x1)
@@ -43,7 +43,7 @@ The Modbus RTU protocol is one of the protocols available within Opta™. In thi
 - STP/UTP 24-18AWG (Unterminated) 100-130Ω rated
 - STP/UTP 22-16AWG (Terminated) 100-130Ω rated
 
-### Software Requirements
+#### Software Requirements
 
 - [Arduino IDE 1.8.10+](https://www.arduino.cc/en/software), [Arduino IDE 2.0+](https://www.arduino.cc/en/software), or [Arduino Web Editor](https://create.arduino.cc/editor)
 - If you choose an offline Arduino IDE, you must install the following libraries: `ArduinoRS485`, and `ArduinoModbus`. You can install these libraries via Library Manager of the Arduino IDE.