Fix flag name referenced in build_event_stream.proto.

The BEP may now be configured to report files inline in the `TargetComplete`
event on a per-output group basis, if the flag
`--experimental_build_event_output_group_mode` is set appropriately.  The
`.proto` file references the correct flag name now.

h/t to @sluongng for pointing out this discrepancy.

RELNOTES: n/a
PiperOrigin-RevId: 693463641
Change-Id: Ie0fec48a94ce665aeeaf0c46cf2bed88fd3fd6ba

src/main/java/com/google/devtools/build/lib/buildeventstream/proto/build_event_stream.proto

@@ -613,7 +613,7 @@ message OutputGroup {
   bool incomplete = 4;
 
   // Inlined files that belong to this output group, requested via
-  // --build_event_inline_output_groups.
+  // --experimental_build_event_output_group_mode.
   repeated File inline_files = 5;
 }