Remove FLAGS_quic_ string from a comment.
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

google/quiche,google/quiche,google/quiche,google/quiche