Update pyspark example (#6264)

The older version seems to be no longer available from mirror.

Pull request: #6264

Author: lefou

example/scalalib/spark/2-hello-pyspark/build.mill

@@ -4,7 +4,7 @@ import mill.*, pythonlib.*
 object foo extends PythonModule {
 
   def mainScript = Task.Source("src/foo.py")
-  def pythonDeps = Seq("pyspark==3.5.4")
+  def pythonDeps = Seq("pyspark==3.5.7")
 
   object test extends PythonTests, TestModule.Unittest