basesink: Fix average-rate value in stats structure

Documentation says "average-rate" is averate frame rate
but priv->avg_rate is not something related to frame rate.
35 jobs for !704 with basesink-fix-stats in 25 seconds (queued for 10 seconds)
latest detached
Status Job ID Name Coverage
  Build Docker
passed #5737653
alpine amd64 manifest builder docker

00:00:16

passed #5737652
android docker

00:00:14

passed #5737651
cerbero fedora amd64 docker

00:00:13

passed #5737650
fedora amd64 docker

00:00:13

passed #5737654
gst-indent amd64 docker

00:00:14

 
  Preparation
passed #5737656
gst indent

00:00:08

manual #5737655
manual
manifest
 
  Build
created #5737665
gstreamer
build cerbero cross-android universal
created #5737669
gst-ios-13.2
build cerbero cross-ios universal
created #5737666
gstreamer
build cerbero cross win32
created #5737667
gstreamer
build cerbero cross win64
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gstreamer
build cerbero fedora x86_64
created #5737668
gst-macos-10.15
build cerbero macos x86_64
created #5737670
1809 gstreamer-windows windows docker
build cerbero msvc x86_64
created #5737660
build clang fedora x86_64
created #5737663
1809 windows docker allowed to fail manual
build msys2
created #5737657
build nodebug fedora x86_64
created #5737658
build static fedora x86_64
created #5737659
build static nodebug fedora x86_64
created #5737661
1809 windows docker
build vs2017 amd64
created #5737662
1809 windows docker
build vs2017 x86
 
  Test
created #5737671
gstreamer
check fedora
created #5737672
gstreamer
integration testsuites fedora 1/4
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gstreamer
integration testsuites fedora 2/4
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gstreamer
integration testsuites fedora 3/4
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gstreamer
integration testsuites fedora 4/4
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gstreamer
valgrind bad
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gstreamer
valgrind base
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gstreamer
valgrind core
created #5737681
gstreamer
valgrind ges
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gstreamer
valgrind good
created #5737679
gstreamer
valgrind ugly
 
  Integrate
created #5737682
cross-android universal examples
created #5737684
gst-ios-13.2
cross-ios universal examples
created #5737683
documentation