fix ttyWrite calculation to avoid truncating data

the math was wrong. The easiest way to see this was attempting to paste
a large chunk of text, it'd just skip certain parts and truncate it.

src/Window.zig

@@ -392,7 +392,7 @@ fn queueWrite(self: *Window, data: []const u8) !void {
             ttyWrite,
         );
 
-        i += end;
+        i = end;
     }
 }