Fix Topic metrics documentation (apache#6495)

### Motivation

*Explain here the context, and why you're making that change. What is the problem you're trying to solve.*

Motivation is to have correct reference-metrics documentation.

### Modifications

*Describe the modifications you've done.*

There is an error in the `Topic metrics` section

`pulsar_producers_count` => `pulsar_in_messages_total`

site2/docs/reference-metrics.md

@@ -174,7 +174,7 @@ All the topic metrics are labelled with the following labels:
 | pulsar_storage_write_latency_le_* | Histogram | The entry rate of a topic that the storage write latency is smaller with a given threshold.<br> Available thresholds: <br><ul><li>pulsar_storage_write_latency_le_0_5: <= 0.5ms </li><li>pulsar_storage_write_latency_le_1: <= 1ms</li><li>pulsar_storage_write_latency_le_5: <= 5ms</li><li>pulsar_storage_write_latency_le_10: <= 10ms</li><li>pulsar_storage_write_latency_le_20: <= 20ms</li><li>pulsar_storage_write_latency_le_50: <= 50ms</li><li>pulsar_storage_write_latency_le_100: <= 100ms</li><li>pulsar_storage_write_latency_le_200: <= 200ms</li><li>pulsar_storage_write_latency_le_1000: <= 1s</li><li>pulsar_storage_write_latency_le_overflow: > 1s</li></ul> |
 | pulsar_entry_size_le_* | Histogram | The entry rate of a topic that the entry size is smaller with a given threshold.<br> Available thresholds: <br><ul><li>pulsar_entry_size_le_128: <= 128 bytes </li><li>pulsar_entry_size_le_512: <= 512 bytes</li><li>pulsar_entry_size_le_1_kb: <= 1 KB</li><li>pulsar_entry_size_le_2_kb: <= 2 KB</li><li>pulsar_entry_size_le_4_kb: <= 4 KB</li><li>pulsar_entry_size_le_16_kb: <= 16 KB</li><li>pulsar_entry_size_le_100_kb: <= 100 KB</li><li>pulsar_entry_size_le_1_mb: <= 1 MB</li><li>pulsar_entry_size_le_overflow: > 1 MB</li></ul> |
 | pulsar_in_bytes_total | Counter | The total number of bytes received for this topic |
-| pulsar_producers_count | Counter | The total number of messages received for this topic |
+| pulsar_in_messages_total | Counter | The total number of messages received for this topic |
 
 #### Replication metrics