Help: Fix CPACK_DEBIAN_PACKAGE_SHLIBDEPS_PRIVATE_DIRS documentation

The docs added by commit d586a4ad (CPackDeb: dpkg-shlibdeps now
supports searching for private shared libs, 2021-01-19,
v3.20.0-rc1~115^2) specify the `-d` option, but the implementation
actually uses the `-l` option.

Issue: #21838

Help/cpack_gen/deb.rst

@@ -347,7 +347,7 @@ List of CPack DEB generator specific variables:
  .. versionadded:: 3.20
 
  May be set to a list of directories that will be given to ``dpkg-shlibdeps``
- via its ``-d`` option. These will be searched by ``dpkg-shlibdeps`` in order
+ via its ``-l`` option. These will be searched by ``dpkg-shlibdeps`` in order
  to find private shared library dependencies.
 
  * Mandatory : NO