docs: fix element names in section headers

Hopefully that'll make hotdoc pick up the docs for these elements.

Part-of: <!1428>
29 jobs for master in 59 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Build Docker
passed #3561454
alpine amd64 manifest builder docker

00:00:25

passed #3561453
android docker

00:00:40

passed #3561452
cerbero fedora amd64 docker

00:00:31

passed #3561451
fedora amd64 docker

00:00:31

passed #3561455
gst-indent amd64 docker

00:00:30

 
  Preparation
passed #3561457
gst indent

00:00:18

manual #3561456
manual
manifest
 
  Build
created #3561466
gstreamer
build cerbero cross-android universal
created #3561470
gst-ios-13.2
build cerbero cross-ios universal
created #3561467
gstreamer
build cerbero cross win32
created #3561468
gstreamer
build cerbero cross win64
created #3561465
gstreamer
build cerbero fedora x86_64
created #3561469
gst-macos-10.15
build cerbero macos x86_64
created #3561461
build clang fedora x86_64
created #3561464
1809 windows docker allowed to fail manual
build msys2
created #3561458
build nodebug fedora x86_64
created #3561459
build static fedora x86_64
created #3561460
build static nodebug fedora x86_64
created #3561462
1809 windows docker
build vs2017 amd64
created #3561463
1809 windows docker
build vs2017 x86
 
  Test
created #3561471
gstreamer
check fedora
created #3561472
gstreamer
integration testsuites fedora 1/4
created #3561473
gstreamer
integration testsuites fedora 2/4
created #3561474
gstreamer
integration testsuites fedora 3/4
created #3561475
gstreamer
integration testsuites fedora 4/4
created #3561476
gstreamer
valgrind bad
 
  Integrate
created #3561477
cross-android universal examples
created #3561479
gst-ios-13.2
cross-ios universal examples
created #3561478
documentation