doc: fix fs.promises sample codes.

doc/api/fs.md

@@ -3453,6 +3453,7 @@ added: v10.0.0
 Closes the file descriptor.
 
 ```js
+const fsPromises = require('fs').promises;
 async function openAndClose() {
   let filehandle;
   try {
@@ -3564,6 +3565,9 @@ For example, the following program retains only the first four bytes of the
 file:
 
 ```js
+const fs = require('fs');
+const fsPromises = fs.promises;
+
 console.log(fs.readFileSync('temp.txt', 'utf8'));
 // Prints: Node.js
 
@@ -3580,6 +3584,9 @@ If the file previously was shorter than `len` bytes, it is extended, and the
 extended part is filled with null bytes (`'\0'`). For example,
 
 ```js
+const fs = require('fs');
+const fsPromises = fs.promises;
+
 console.log(fs.readFileSync('temp.txt', 'utf8'));
 // Prints: Node.js
 
@@ -3684,6 +3691,9 @@ with an `Error` object. The following example checks if the file
 `/etc/passwd` can be read and written by the current process.
 
 ```js
+const fs = require('fs');
+const fsPromises = fs.promises;
+
 fsPromises.access('/etc/passwd', fs.constants.R_OK | fs.constants.W_OK)
   .then(() => console.log('can access'))
   .catch(() => console.error('cannot access'));
@@ -3776,7 +3786,7 @@ then the operation will fail.
 Example:
 
 ```js
-const fs = require('fs');
+const fsPromises = require('fs').promises;
 
 // destination.txt will be created or overwritten by default.
 fsPromises.copyFile('source.txt', 'destination.txt')
@@ -3789,6 +3799,7 @@ following example.
 
 ```js
 const fs = require('fs');
+const fsPromises = fs.promises;
 const { COPYFILE_EXCL } = fs.constants;
 
 // By using COPYFILE_EXCL, the operation will fail if destination.txt exists.