Fix video simple snippet (2) (rerun-io#7315)

### What

There was another ancient copy-pasta mistake where the `MediaType` was
being set to `glb` for `video/mp4`...

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* Using examples from latest `main` build:
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rerun_cpp/src/rerun/archetypes/asset_video_ext.cpp

@@ -73,7 +73,7 @@ namespace rerun::archetypes {
         std::transform(ext.begin(), ext.end(), ext.begin(), ::tolower);
 
         if (ext == ".mp4") {
-            return rerun::components::MediaType::glb();
+            return rerun::components::MediaType::mp4();
         } else {
             return std::nullopt;
         }