Fix a typo in walsender.c

Hi,

Attached a minor patch for variable name in comment:  
s/progress_update/update_progress

   ---include/server/replication/logical.h
   ...
   35 typedef struct LogicalDecodingContext
   36 {
   ...
   68 LogicalOutputPluginWriterUpdateProgress update_progress;

Regards,

-- 
Atsushi Torikoshi
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center