doc: fix "added in" for Buffer.allocUnsafeSlow()

PR-URL: https://github.com/nodejs/node/pull/15330
Fixes: https://github.com/nodejs/node/issues/15279
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/buffer.md

@@ -616,7 +616,7 @@ additional performance that [`Buffer.allocUnsafe()`] provides.
 
 ### Class Method: Buffer.allocUnsafeSlow(size)
 <!-- YAML
-added: v5.10.0
+added: v5.12.0
 -->
 
 * `size` {integer} The desired length of the new `Buffer`