fs/procfs: fix output format of fd info

Refered to PRINTF(3), the [v]snprintf returns the number of characters
printed (excluding the null byte used to end output to strings).

Signed-off-by: wangjianyu3 <wangjianyu3@xiaomi.com>

fs/procfs/fs_procfsproc.c

@@ -1330,7 +1330,7 @@ static ssize_t proc_groupfd(FAR struct proc_file_s *procfile,
                                        fdp->f_backtrace,
                                        CONFIG_FS_BACKTRACE);
 #endif
-          procfile->line[linesize - 2] = '\n';
+          procfile->line[linesize - 1] = '\n';
         }
 
       file_put(filep);