doc: improve prepositions in buffer.md

* `at which to begin copying to` -> `at which to begin writing`
* `at which to begin copying from` -> `from which to begin copying`
* wrap at 80 chars

PR-URL: #19817
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/buffer.md

@@ -1039,8 +1039,8 @@ added: v0.1.90
 
 * `target` {Buffer|Uint8Array} A `Buffer` or [`Uint8Array`] to copy into.
 * `targetStart` {integer} The offset within `target` at which to begin
-  copying to. **Default:** `0`.
-* `sourceStart` {integer} The offset within `buf` at which to begin copying from.
+  writing. **Default:** `0`.
+* `sourceStart` {integer} The offset within `buf` from which to begin copying.
   **Default:** `0`.
 * `sourceEnd` {integer} The offset within `buf` at which to stop copying (not
   inclusive). **Default:** [`buf.length`].