Update log data model JSON examples to use nanosecond values

The data model says that timestamps are nanosecond values, but examples use milliseconds.
Updated to use stringified nanoseconds according to open-telemetry#1637

Fixes: open-telemetry#1049

specification/logs/data-model.md

@@ -450,8 +450,7 @@ Example 1
 
 ```javascript
 {
-  "Timestamp": 1586960586000, // JSON needs to make a decision about
-                              // how to represent nanoseconds.
+  "Timestamp": "1586960586000000000",
   "Attributes": {
     "http.status_code": 500,
     "http.url": "http://example.com",
@@ -475,7 +474,7 @@ Example 2
 
 ```javascript
 {
-  "Timestamp": 1586960586000,
+  "Timestamp": "1586960586000000000",
   ...
   "Body": {
     "i": "am",
@@ -491,7 +490,7 @@ Example 3
 
 ```javascript
 {
-   "Timestamp": 1586960586000,
+   "Timestamp": "1586960586000000000",
    "Attributes":{
       "http.scheme":"https",
       "http.host":"donut.mycie.com",