qtplasmac docs: update python serial version

Author: snowgoer540

docs/src/plasma/qtplasmac.adoc

@@ -4007,7 +4007,7 @@ If pyserial is not installed an error message will be displayed.
 
 To install pyserial, enter the following command into a terminal window:
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-sudo apt install python-serial
+sudo apt install python3-serial
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 A typical <<plasma:rs485_connections,connection diagram>> is shown in the appendix of this document as well as confirmed working interfaces.