server : update help metrics processing/deferred

This commit updates the help text for the metrics `requests_processing`
and `requests_deferred` to be more grammatically correct.

Currently the returned metrics look like this:
```console
\# HELP llamacpp:requests_processing Number of request processing.
\# TYPE llamacpp:requests_processing gauge
llamacpp:requests_processing 0
\# HELP llamacpp:requests_deferred Number of request deferred.
\# TYPE llamacpp:requests_deferred gauge
llamacpp:requests_deferred 0
```

With this commit, the metrics will look like this:
```console
\# HELP llamacpp:requests_processing Number of requests processing.
\# TYPE llamacpp:requests_processing gauge
llamacpp:requests_processing 0
\# HELP llamacpp:requests_deferred Number of requests deferred.
\# TYPE llamacpp:requests_deferred gauge
llamacpp:requests_deferred 0
```
This is also consistent with the description of the metrics in the
server examples [README.md](https://github.com/ggerganov/llama.cpp/tree/master/examples/server#get-metrics-prometheus-compatible-metrics-exporter).

Author: danbev

examples/server/server.cpp

@@ -3575,11 +3575,11 @@ int main(int argc, char ** argv) {
                     {"value",  (uint64_t) res_metrics->kv_cache_tokens_count}
             },{
                     {"name",  "requests_processing"},
-                    {"help",  "Number of request processing."},
+                    {"help",  "Number of requests processing."},
                     {"value",  (uint64_t) res_metrics->n_processing_slots}
             },{
                     {"name",  "requests_deferred"},
-                    {"help",  "Number of request deferred."},
+                    {"help",  "Number of requests deferred."},
                     {"value",  (uint64_t) res_metrics->n_tasks_deferred}
             }}}
         };