docs: update changelog.md

Author: futsuuu

changelog.md

@@ -11,6 +11,7 @@
 * `FIX` Typed `@field` (eg `---@field [string] boolean`) should not override other defined field [#2171](https://github.com/LuaLS/lua-language-server/issues/2171), [#2711](https://github.com/LuaLS/lua-language-server/issues/2711)
 * `FIX` don't return empty hover doc when luals failed to find definition
 * `FIX` Prevent stack overflow when attempting to resolve function return values. [#3246](https://github.com/LuaLS/lua-language-server/issues/3246)
+* `FIX` Fix type annotations for `string.buffer` module
 
 ## 3.15.0
 `2025-6-25`