Fix cross-crate documentation link to objc2::topics #3249

Job	Run time
Check MSRV	59s
Check formatting	9s
Lint macOS x86_64	3m 29s
Lint iOS ARMv7s	3m 15s
Lint visionOS Aarch64 simulator	3m 17s
Lint GNUStep + exceptions	1m 21s
Lint header-translator	31s
Lint test-assembly	25s
Test macOS 15 Aarch64	1m 50s
Assembly tests	53s
${{ ((matrix.test && 'Test') \|\| 'Build') }} ${{ matrix.name }}	0s
Check framework features	0s
Test Compiler-RT	26s
Verify header translator output	1m 48s
Test on iOS simulator w. dinghy	0s
Test GNUStep with libobjc2 v1.9	59s
Test GNUStep with libobjc2 v2.0	1m 14s
Compiler UI	47s
Test GNUStep with libobjc2 v2.1 on nightly	1m 1s
Test GNUStep 32bit	1m 17s
	23m 41s

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