chore: Bump the default split page concurrency (#122)

Verified that this shows a speedup by doing a local pip install and
running the following snippet before and after the change:

```
from unstructured_client import UnstructuredClient
from unstructured_client.models import shared

s = UnstructuredClient(
    server_url=SERVER_URL,
    api_key_auth=API_KEY,
    )

filename = "../_sample_docs/layout-parser-paper.pdf"

with open(filename, "rb") as f:
    # Note that this currently only supports a single file
    files=shared.Files(
        content=f.read(),
        file_name=filename,
	)

req = shared.PartitionParameters(
    files=files,
    strategy="hi_res",
)

start_time = time.time()
resp = s.general.partition(req)
end_time = time.time()
print(f"Elapsed time: {end_time - start_time} seconds")
```

src/unstructured_client/_hooks/custom/split_pdf_hook.py

@@ -36,7 +36,7 @@
 
 
 DEFAULT_STARTING_PAGE_NUMBER = 1
-DEFAULT_CONCURRENCY_LEVEL = 5
+DEFAULT_CONCURRENCY_LEVEL = 8
 MAX_CONCURRENCY_LEVEL = 15
 MIN_PAGES_PER_SPLIT = 2
 MAX_PAGES_PER_SPLIT = 20