fixup: rephrase mapping[0]

Signed-off-by: Florian Lehner <[email protected]>

opentelemetry/proto/profiles/v1development/pprofextended.proto

@@ -80,7 +80,7 @@ message Profile {
  // Mapping from address ranges to the image/binary/library mapped
  // into that address range. mapping[0] will be the main binary.
  // If multiple binaries contribute to the Profile and no main
- // binary can be identified, mapping is optional.
+ // binary can be identified, mapping[0] has no special meaning.
  repeated Mapping mapping = 3;
  // Locations referenced by samples via location_indices.
  repeated Location location = 4;