[SPARK-25909] fix documentation on cluster managers

## What changes were proposed in this pull request?

Propose changing the documentation to state that there are 4, not 3, cluster managers available.

## How was this patch tested?

This is a docs-only patch and doesn't need any new testing beyond the normal CI process for Spark.

Closes apache#22922 from jameslamb/bugfix/cluster_docs.

Authored-by: James Lamb <[email protected]>
Signed-off-by: Sean Owen <[email protected]>

Author: jameslamb

docs/cluster-overview.md

@@ -45,7 +45,7 @@ There are several useful things to note about this architecture:
 
 # Cluster Manager Types
 
-The system currently supports three cluster managers:
+The system currently supports several cluster managers:
 
 * [Standalone](spark-standalone.html) -- a simple cluster manager included with Spark that makes it
   easy to set up a cluster.