Update location of 'enable disk backend for full preview cache' preference in lighttable docs

The preference has moved, its documentation has been updated, but the link from the lighttable page has been forgotten.

Author: kofa73

content/lighttable/lighttable-modes/culling.md

@@ -36,6 +36,6 @@ dynamic mode
 
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-**Hint:** To enhance performance when loading zoomed images, you can enable ([preferences > processing > cpu/gpu/memory > enable disk backend for full preview cache](../../preferences-settings/processing.md#cpu--gpu--memory)). Bear in mind that this can occupy a lot of disk space.
+**Hint:** To enhance performance when loading zoomed images, you can enable ([preferences > lighttable > thumbnails > enable disk backend for full preview cache](../../preferences-settings/lighttable.md#thumbnails)). Bear in mind that this can occupy a lot of disk space.
 
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