doc: use lowercase for zlib

PR-URL: #25371
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/index.md

@@ -56,7 +56,7 @@
 * [V8](v8.html)
 * [VM](vm.html)
 * [Worker Threads](worker_threads.html)
-* [ZLIB](zlib.html)
+* [Zlib](zlib.html)
 
 <div class="line"></div>