Help: Make cmake-buildsystem(7) header-only examples consistent

Extend commit 93696d0f2e (Help: Restore cmake-buildsystem(7) header-only
library example, 2023-01-13, v3.25.2~3^2) to cover a second part of the
example from commit 4b25a0d512 (Help/manual: Update header-only library
example, 2022-04-22, v3.24.0-rc1~220^2).

Issue: #24319

Help/manual/cmake-buildsystem.7.rst

@@ -1513,7 +1513,7 @@ default header set along with the target:
 
   add_library(Eigen INTERFACE)
 
-  target_sources(Eigen INTERFACE
+  target_sources(Eigen PUBLIC
     FILE_SET HEADERS
       BASE_DIRS src
       FILES src/eigen.h src/vector.h src/matrix.h