Fix parameter name in exception.

csharp/src/Google.Protobuf/Collections/MapField.cs

diff --git a/csharp/src/Google.Protobuf/Collections/MapField.cs b/csharp/src/Google.Protobuf/Collections/MapField.cs
index 004ff54..0fa63be 100644
--- a/csharp/src/Google.Protobuf/Collections/MapField.cs
+++ b/csharp/src/Google.Protobuf/Collections/MapField.cs
@@ -737,7 +737,7 @@
             {
                 if (index < 0)
                 {
-                    throw new ArgumentOutOfRangeException("arrayIndex");
+                    throw new ArgumentOutOfRangeException("index");
                 }
                 if (index + Count >= array.Length)
                 {