Fixed npm publish appending multiple copies into .mjs

Change-Id: Id8c373b0f77d9a0f4922257339abb4e01e06c2d4

package.json

 {
   "name": "flatbuffers",
-  "version": "1.10.0",
+  "version": "1.10.2",
   "description": "Memory Efficient Serialization Library",
   "files": ["js/flatbuffers.js", "js/flatbuffers.mjs"],
   "main": "js/flatbuffers",
@@ -11,7 +11,7 @@
   },
   "scripts": {
     "test": "tests/JavaScriptTest.sh",
-    "append-esm-export": "sed \"s/this.flatbuffers = flatbuffers;/export { flatbuffers };/\" js/flatbuffers.js >> js/flatbuffers.mjs",
+    "append-esm-export": "sed \"s/this.flatbuffers = flatbuffers;/export { flatbuffers };/\" js/flatbuffers.js > js/flatbuffers.mjs",
     "prepublishOnly": "npm run append-esm-export"
   },
   "repository": {