Docs: Fix typo in last line of Vinyl usage example (#2382)

Author: rninne

docs/api/vinyl.md

@@ -40,7 +40,7 @@ file.stem = 'foo';
 file.path === '/specs/foo.txt';
 file.extname === '.txt';
 file.extname = '.js';
-file.path === '/specs/file.js';
+file.path === '/specs/foo.js';
 ```
 
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