[SPARK-26807][DOCS] Clarify that Pyspark is on PyPi now

## What changes were proposed in this pull request?

Docs still say that Spark will be available on PyPi "in the future"; just needs to be updated.

## How was this patch tested?

Doc build

Closes #23933 from srowen/SPARK-26807.

Authored-by: Sean Owen <sean.owen@databricks.com>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

Author: srowen

docs/index.md

@@ -20,7 +20,7 @@ Please see [Spark Security](security.html) before downloading and running Spark.
 Get Spark from the [downloads page](https://spark.apache.org/downloads.html) of the project website. This documentation is for Spark version {{site.SPARK_VERSION}}. Spark uses Hadoop's client libraries for HDFS and YARN. Downloads are pre-packaged for a handful of popular Hadoop versions.
 Users can also download a "Hadoop free" binary and run Spark with any Hadoop version
 [by augmenting Spark's classpath](hadoop-provided.html).
-Scala and Java users can include Spark in their projects using its Maven coordinates and in the future Python users can also install Spark from PyPI.
+Scala and Java users can include Spark in their projects using its Maven coordinates and Python users can install Spark from PyPI.
 
 
 If you'd like to build Spark from