doc: fix YAML syntax in fs.md

Fixes YAML parsing issues introduced in c8a2143.

PR-URL: https://github.com/nodejs/node/pull/15769
Refs: https://github.com/nodejs/node/pull/15680
Reviewed-By: Lance Ball <lball@redhat.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/fs.md

@@ -2341,8 +2341,8 @@ added: v0.4.2
 changes:
   - version: v8.0.0
     pr-url: https://github.com/nodejs/node/pull/11919
-    description: `NaN`, `Infinity`, and `-Infinity` are no longer valid time
-                 specifiers.
+    description: "`NaN`, `Infinity`, and `-Infinity` are no longer valid time
+                 specifiers."
   - version: v7.6.0
     pr-url: https://github.com/nodejs/node/pull/10739
     description: The `path` parameter can be a WHATWG `URL` object using `file:`
@@ -2377,8 +2377,8 @@ added: v0.4.2
 changes:
   - version: v8.0.0
     pr-url: https://github.com/nodejs/node/pull/11919
-    description: `NaN`, `Infinity`, and `-Infinity` are no longer valid time
-                 specifiers.
+    description: "`NaN`, `Infinity`, and `-Infinity` are no longer valid time
+                 specifiers."
   - version: v7.6.0
     pr-url: https://github.com/nodejs/node/pull/10739
     description: The `path` parameter can be a WHATWG `URL` object using `file:`