docs: enable syntax highlighting to a code block

docs/JestObjectAPI.md

@@ -147,7 +147,7 @@ Creates a new property with the same primitive value as the original property.
 
 Example:
 
-```
+```js
 // example.js
 module.exports = {
   function: function square(a, b) {

website/versioned_docs/version-25.x/JestObjectAPI.md

@@ -143,7 +143,7 @@ Creates a new property with the same primitive value as the original property.
 
 Example:
 
-```
+```js
 // example.js
 module.exports = {
   function: function square(a, b) {

website/versioned_docs/version-26.x/JestObjectAPI.md

@@ -147,7 +147,7 @@ Creates a new property with the same primitive value as the original property.
 
 Example:
 
-```
+```js
 // example.js
 module.exports = {
   function: function square(a, b) {

website/versioned_docs/version-27.0/JestObjectAPI.md

@@ -147,7 +147,7 @@ Creates a new property with the same primitive value as the original property.
 
 Example:
 
-```
+```js
 // example.js
 module.exports = {
   function: function square(a, b) {