texlab: Fix test for case-insensitive filesystems (#48027)

- On Linux, `require 'Open3'` failed, but `require 'open3'` succeeded.
- So, upstream this change just in case and to make merges easier.

Formula/texlab.rb

@@ -19,7 +19,7 @@ class Texlab < Formula
   end
 
   test do
-    require "Open3"
+    require "open3"
 
     begin
       stdin, stdout, _, wait_thr = Open3.popen3("#{bin}/texlab")