Help: Restore cmake-buildsystem(7) header-only library example

Since commit 4391913133 (Add INTERFACE libraries to generated
buildsystem if they have SOURCES, 2020-07-20, v3.19.0-rc1~346^2~1) the
"Eigen" example in the `cmake-buildsystem(7)` manual is supposed to show
a header-only library in which the headers are attached as sources to be
edited in IDEs.  This was accidentally broken by commit 4b25a0d512
(Help/manual: Update header-only library example, 2022-04-22,
v3.24.0-rc1~220^2) when updating the example to use a `FILE_SET`.

Fixes: #24319

Help/manual/cmake-buildsystem.7.rst

@@ -1047,7 +1047,7 @@ them to a header set using the :command:`target_sources` command:
 
   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