✏️ Fix wrong example snippet for M109

[antedeguemon]

Hey there! 🖖

The M109 gcode documentation states:

[R<temp>]
Target temperature (wait for cooling or heating).

[S<temp>]
Target temperature (wait only when heating).

But there is an example on the same page where it says the R will wait if heating up, not mentioning the or cooling part of the conditional:

Set target temperature for E1 and wait (if heating up)
M109 T1 R205

I believe S<temp> was meant instead of R<temp>, or the or cooling part was forgotten. This pull request assumes the first scenario and replaces S<temp> ➡️ R<temp>.

[thinkyhead]

Thanks for the fix!