rtpsession: fix comment to refer to buffers instead of groups

One comments in gst_rtp_session_chain_send_rtp_common() is referring to
groups in a buffer list, however this concept of "group" comes from
GStreamer 0.10 and does not exist anymore in GStreamer 1.0, so update the
comment to refer to buffers instead.
13 jobs for <span class="ref-name">rtpmanager-misc-fixes</span> in 89 minutes and 1 second (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #208741
gst indent

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passed #208740
manifest

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  Build
passed #208742
build fedora x86_64

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passed #208743
build nodebug fedora x86_64

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  Test
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check fedora

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  Full Builds
passed #209017
build cerbero android universal

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passed #208747
build cerbero cross win32

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passed #208748
build cerbero cross win64

00:39:51

passed #208745
build cerbero fedora x86_64

00:32:00

failed #208746
build cerbero android universal

01:00:00

 
  Full Tests
passed #208750
valgrind bad

00:07:05

passed #208749
valgrind good

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  Apps
passed #208751
android universal examples

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