doc: modernize child_process example code

1. equal => strictEqual.
2. let => const for the variable that is not reassigned.
3. fix spaces.
4. stringify erroneous raw buffer outputs.
5. fix a typo.

PR-URL: https://github.com/nodejs/node/pull/10102
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

doc/api/child_process.md

@@ -91,11 +91,11 @@ const spawn = require('child_process').spawn;
 const bat = spawn('cmd.exe', ['/c', 'my.bat']);
 
 bat.stdout.on('data', (data) => {
-  console.log(data);
+  console.log(data.toString());
 });
 
 bat.stderr.on('data', (data) => {
-  console.log(data);
+  console.log(data.toString());
 });
 
 bat.on('exit', (code) => {
@@ -383,7 +383,7 @@ ps.on('close', (code) => {
 });
 
 grep.stdout.on('data', (data) => {
-  console.log(`${data}`);
+  console.log(data.toString());
 });
 
 grep.stderr.on('data', (data) => {
@@ -850,7 +850,7 @@ as in this example:
 'use strict';
 const spawn = require('child_process').spawn;
 
-let child = spawn('sh', ['-c',
+const child = spawn('sh', ['-c',
   `node -e "setInterval(() => {
       console.log(process.pid, 'is alive')
     }, 500);"`
@@ -1098,20 +1098,20 @@ const child_process = require('child_process');
 
 const child = child_process.spawn('ls', {
   stdio: [
-      0, // Use parents stdin for child
+    0, // Use parent's stdin for child
     'pipe', // Pipe child's stdout to parent
     fs.openSync('err.out', 'w') // Direct child's stderr to a file
   ]
 });
 
-assert.equal(child.stdio[0], null);
+assert.strictEqual(child.stdio[0], null);
-assert.equal(child.stdio[0], child.stdin);
+assert.strictEqual(child.stdio[0], child.stdin);
 
 assert(child.stdout);
-assert.equal(child.stdio[1], child.stdout);
+assert.strictEqual(child.stdio[1], child.stdout);
 
-assert.equal(child.stdio[2], null);
+assert.strictEqual(child.stdio[2], null);
-assert.equal(child.stdio[2], child.stderr);
+assert.strictEqual(child.stdio[2], child.stderr);
 ```
 
 ### child.stdout