The declaration of the flush function is obsolete

The flush function in the example is defined as follows:
```
static void cb_stdout_flush(const void *data, size_t bytes,
const char *tag, int tag_len,
struct flb_input_instance *i_ins,
void *out_context,
struct flb_config *config)
```
But in the latest flunt-bit source code, the declaration of the flush function is:
https://github.com/fluent/fluent-bit/blob/master/include/fluent-bit/flb_output.h#L196
```
void (*cb_flush) (struct flb_event_chunk *,
struct flb_output_flush *,
struct flb_input_instance *,
void *,
struct flb_config *);
```

the function definitions in the examples need to be updated to avoid ambiguity for plugin developers.