Update LinuxCNC RTAI Install instructions

Add purge of uspace version to command list

Author: andypugh

docs/src/getting-started/getting-linuxcnc.adoc

@@ -357,9 +357,11 @@ sudo ./linuxcnc-install.sh
 ----
 sudo apt-get update
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-. Install the new realtime kernel, RTAI and the rtai version of linuxcnc.
+. Remove the existing uspace version of LinuxCNC and install the new realtime kernel, RTAI and the rtai version of linuxcnc.
 +
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+sudo apt-get purge linuxcnc-uspace
+sudo apt-get purge linuxcnc-doc*
 sudo apt-get install linuxcnc
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 Reboot the machine, ensuring that the system boots from the new 5.4.258-rtai