doc: change v-notation for version in buffer.md

Change v6 to 6.0.0. We abandoned v-notation for versions to avoid
confusion between v8 (version 8.0.0) and V8 (the JavaScript engine).

PR-URL: nodejs#19567
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

Author: Trott

doc/api/buffer.md

@@ -48,7 +48,7 @@ const buf6 = Buffer.from('tést', 'latin1');
 
 ## `Buffer.from()`, `Buffer.alloc()`, and `Buffer.allocUnsafe()`
 
-In versions of Node.js prior to v6, `Buffer` instances were created using the
+In versions of Node.js prior to 6.0.0, `Buffer` instances were created using the
 `Buffer` constructor function, which allocates the returned `Buffer`
 differently based on what arguments are provided: