fix stderr_logfile_backups example

sample.conf

@@ -59,7 +59,7 @@ serverurl=unix:///tmp/supervisor.sock ; use a unix:// URL  for a unix socket
 ;stdout_capturefile=NONE       ; stdout capturemode data file (default AUTO)
 ;stderr_logfile=/a/path        ; stderr log path, NONE for none; default AUTO
 ;stderr_logfile_maxbytes=1MB   ; max # logfile bytes b4 rotation (default 50MB)
-;stderr_logfile_backups        ; # of stderr logfile backups (default 10)
+;stderr_logfile_backups=10     ; # of stderr logfile backups (default 10)
 ;stderr_capturefile=NONE       ; stderr capturemode data file (default AUTO)
 ;environment=A=1,B=2           ; process environment additions