Fix typo when identifying KLV packets as SMPTE336M-encoded

[ja-ford]

Fixes bluenviron/mediamtx#4848

The encoding name used to identify and mark KLV packets appropriately was typo'd in a few places as "smtpe" ("tpe" vs "pte"). This was causing downstream consumers (including ffmpeg and gstreamer) to not properly identify KLV data streams, and may have caused issues with mediamtx itself identifying KLV data streams (unconfirmed). This typo was introduced in #808

This PR fixes the typo and adjusts casing to stay consistent with other encoding-name mappings in this library.

Testing

We tested this fix locally by serving an MPEGTS file with KLV data via an RTSP stream directly in mediamtx and receiving the stream in gstreamer:

gst-launch-1.0 rtspsrc location="rtsp://127.0.0.1:8554/stream" ! rtpklvdepay ! fakesink

The rtpklvdepay element successfully identifies KLV packets from the stream after this fix.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.92%. Comparing base (6add580) to head (85c6afd).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #877      +/-   ##
==========================================
- Coverage   80.94%   80.92%   -0.03%     
==========================================
  Files         128      128              
  Lines       14323    14323              
==========================================
- Hits        11594    11591       -3     
- Misses       2204     2206       +2     
- Partials      525      526       +1     

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[aler9]

merged, thank you very much!