DOCSP-16458 Add wording for spark 3.1 support (#72)

Author: kanchana-mongodb

source/index.txt

@@ -25,7 +25,7 @@ versions of Apache Spark and MongoDB:
      - MongoDB Version
 
    * - **{+current-version+}**
-     - **3.0.x**
+     - **3.0.x**, **3.1.x**
      - **2.6 or later**
 
    * - 2.4.3