User guide: the OpenFX install paths now include Linux

The Linux OpenFX build ships as of this release, and the guide listed only the
macOS and Windows plugin directories -- not a false claim, but an incomplete
one for anyone who has just downloaded the Linux artefact. Adds
/usr/OFX/Plugins alongside them.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: stoatworks-labs

docs/USER-GUIDE.md

@@ -146,6 +146,7 @@ and restart the host:
 ```
 macOS    /Library/OFX/Plugins/
 Windows  C:\Program Files\Common Files\OFX\Plugins\
+Linux    /usr/OFX/Plugins/
 ```
 
 One difference worth knowing: Persistence is rebuilt from previous frames each