docs: fix malformed hyperlink syntax

Author: marquiz

docs/usage/custom-resources.md

@@ -93,7 +93,7 @@ default feature labels generated by NFD. The sample rules can be used as a base
 to customize NFD feature labels. To use them in place of the the NFD built-in
 labels, the corresponding feature source(s) of nfd-worker should be disabled
 with the
-(`core.labelSources`)[../reference/worker-configuration-reference.md#corelabelsources])
+[`core.labelSources`](../reference/worker-configuration-reference.md#corelabelsources)
 configuration option.
 
 ## NodeResourceTopology