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
37 jobs for !5835 with doc-cpack-dpkg-shlibdeps-l in 3 minutes and 58 seconds (queued for 1 second)
latest detached
Status Job ID Name Coverage
  Build
manual #5549836
linux docker cmake build manual
build:centos6-x86_64
passed #5549833
linux docker cmake build
build:debian10-iwyu

00:01:25

manual #5549837
linux docker cmake build linux-3.17 manual
build:fedora33-ninja
passed #5549835
linux docker cmake build linux-3.17
build:fedora33-sphinx

00:03:58

passed #5549834
linux docker cmake build linux-3.17
build:fedora33-tidy

00:03:58

manual #5549839
cmake macos shell nonconcurrent xcode-12.3 manual
build:macos-makefiles
manual #5549838
cmake macos shell nonconcurrent xcode-12.3 manual
build:macos-ninja
manual #5549840
windows cmake shell vs2019 nonconcurrent msvc-19.28 manual
build:windows-vs2019-x64-ninja
 
  Test
created #5549841
linux docker cmake x11 linux-3.17
test:fedora33-ninja
created #5549843
cmake macos shell nonconcurrent xcode-12.3
test:macos-makefiles
created #5549842
cmake macos shell nonconcurrent xcode-12.3
test:macos-ninja
created #5549844
windows cmake shell vs2019 nonconcurrent msvc-19.28
test:windows-vs2019-x64-ninja
 
  Test Ext
created #5549847
linux docker cmake cuda-rt
test:cuda10.2-nvidia
created #5549845
linux docker cmake build linux-3.17
test:debian10-ninja
created #5549846
linux docker cmake build linux-3.17
test:fedora33-makefiles
created #5549848
linux docker cmake build linux-3.17
test:fedora33-ninja-multi
created #5549849
cmake macos shell concurrent xcode-12.3
test:macos-xcode
created #5549850
windows cmake shell vs2019 concurrent msvc-19.28
test:windows-vs2019-x64
 
  External
passed cdash

passed ghostflow-check-master

passed ghostflow-check-release

passed ghostflow-stager

passed cdash

passed cdash

passed cdash

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passed cdash

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager