fix: Correct error message about send queue being full (#3593)

## Purpose of this PR

Error message you get when the UTP send queue is full would indicate to
increase the 'Max Send Queue Size' parameter. But it really should be
the 'Max Packet Queue Size'. The send queue size is not something users
should have to modify since we increase dynamically as needed. The
packet queue size however should be increased by users if needed since
we can't increase it dynamically (at least with current UTP).

### Changelog

Didn't seem worthy of a changelog entry. Can add one if requested.

## Testing & QA

No need to test anything. It's just a string change.

## Documentation

No documentation changes necessary.

## Backport

#3594

Co-authored-by: Unity Netcode CI <[email protected]>
Co-authored-by: Noel Stephens <[email protected]>

Author: 3 people

com.unity.netcode.gameobjects/Runtime/Transports/UTP/UnityTransport.cs

@@ -1749,7 +1749,7 @@ internal static FixedString128Bytes ErrorToFixedString(int error)
                 case TransportError.NetworkPacketOverflow:
                     return "packet is too large for the transport (likely need to increase MTU)";
                 case TransportError.NetworkSendQueueFull:
-                    return "send queue full (need to increase 'Max Send Queue Size' parameter)";
+                    return "send queue full (need to increase 'Max Packet Queue Size' parameter)";
                 default:
                     return FixedString.Format("unexpected error code {0}", error);
             }