fix: add missing optional chaining operator

Author: jakecastelli

doc/api/fs.md

@@ -7638,7 +7638,7 @@ try {
   fd = await open('/open/some/file.txt', 'r');
   // Do something with the file
 } finally {
-  await fd.close();
+  await fd?.close();
 }
 ```
 
@@ -7652,7 +7652,7 @@ try {
   fd = await open('file.txt', 'r');
   // Do something with the file
 } finally {
-  await fd.close();
+  await fd?.close();
 }
 ```
 
@@ -7767,7 +7767,7 @@ try {
   fd = await open(Buffer.from('/open/some/file.txt'), 'r');
   // Do something with the file
 } finally {
-  await fd.close();
+  await fd?.close();
 }
 ```