doc: correct introduced_in metadata for buffer doc

The buffer module was introduced in v0.1.90. Updated `introduced_in`
value in `buffer.md` to reflect this.

PR-URL: #19545
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

doc/api/buffer.md

@@ -1,6 +1,6 @@
 # Buffer
 
-<!--introduced_in=v0.10.0-->
+<!--introduced_in=v0.1.90-->
 <!--lint disable maximum-line-length-->
 
 > Stability: 2 - Stable