Update extended3mx.md

Author: GitGaoXing

doc/extended3mx.md

@@ -93,7 +93,7 @@
 "navigationMode":	"ORBIT"
 }],
 "layers":	[{
-"type":	"modelPyramid",
+"type":	"meshPyramid",
 "id":	"model0",
 "name":	"Root",
 "description":	"Converted by ProjSEED/lodToolkit, copyright <a href='https://github.com/ProjSEED/lodToolkit' target='_blank'>ProjSEED</a>.",
@@ -103,4 +103,4 @@
 "root":	"Data/Root.3mxb"
 }]
 }
-```
+```