Fix error on appending arg set when current length equals to capacity in string formatter.

Assets/Plugins/StringFormatter/StringFormatter.cs

@@ -400,7 +400,7 @@ namespace System.Text.Formatting {
                 var segmentsLeft = false;
                 var prevArgIndex = 0;
                 do {
-                    CheckCapacity((int)(end - curr));
+                    CheckCapacity((int)(end - curr) + 1);
                     fixed (char* bufferPtr = &buffer[currentCount])
                         segmentsLeft = AppendSegment(ref curr, end, bufferPtr, ref prevArgIndex, ref args);
                 }