fix: correct relative path to actors.md in gather operator docs

Motivation:
The Paradox doc build fails with "Unknown page [../../actors.md]"
because gather.md is three directory levels below the paradox root.

Modification:
Change ../../actors.md to ../../../actors.md, matching the pattern
used in neighboring ask.md.

Result:
Paradox doc build succeeds.

Tests:
Not run - docs only (path fix verified by comparing ask.md in same directory)

References:
Refs #2848

Author: He-Pin

docs/src/main/paradox/stream/operators/Source-or-Flow/gather.md

@@ -23,7 +23,7 @@ upstream failure, downstream cancellation, abrupt stage termination, or supervis
 Elements emitted from `onComplete` are emitted before upstream-failure propagation, completion, or restart,
 and are ignored on downstream cancellation and abrupt termination.
 
-The `gather` operator adheres to the @ref:[ActorAttributes.SupervisionStrategy](../../actors.md) attribute.
+The `gather` operator adheres to the @ref:[ActorAttributes.SupervisionStrategy](../../../actors.md) attribute.
 
 For a simpler stateless mapping, use @ref:[map](map.md) or @ref:[mapConcat](mapConcat.md).