Remove FLAGS_quic_ string from a comment.

Presumbit rule added to quic/METADATA at cl/253074929 prevents me from making
any changes to quic_stream.h because the string "FLAGS_quic_" appears in a
comment.  This CL changes that to a syntax that matches a dozen other comments
in quic/core, see
cs/f://depot/google3/third_party/quic/%20%22latched%20value%20of%22

gfe-relnote: n/a, comment-only change.
PiperOrigin-RevId: 253765056
Change-Id: Ie6ef7fba3da56fddcf06d49870304d4914d28b6d

quic/core/quic_stream.h

diff --git a/quic/core/quic_stream.h b/quic/core/quic_stream.h
index a9d7703..5ae9d32 100644
--- a/quic/core/quic_stream.h
+++ b/quic/core/quic_stream.h
@@ -529,7 +529,7 @@
   // or discarded.
   QuicStreamSendBuffer send_buffer_;
 
-  // Latched value of FLAGS_quic_buffered_data_threshold.
+  // Latched value of quic_buffered_data_threshold.
   const QuicByteCount buffered_data_threshold_;
 
   // If true, then this stream has precedence over other streams for write