doc fix for SQL DecimalType

Per @marmbrus’s instructions here:
apache#2367 (comment)

Author: nchammas

docs/sql-programming-guide.md

@@ -1109,7 +1109,7 @@ evaluated by the SQL execution engine.  A full list of the functions supported c
     The range of numbers is from `-9223372036854775808` to `9223372036854775807`.
     - `FloatType`: Represents 4-byte single-precision floating point numbers.
     - `DoubleType`: Represents 8-byte double-precision floating point numbers.
-    - `DecimalType`: Represents 16-byte arbitrary-precision floating point numbers.
+    - `DecimalType`: Represents arbitrary-precision signed decimal numbers. Backed internally by `java.math.BigDecimal`. A `BigDecimal` consists of an arbitrary precision integer unscaled value and a 32-bit integer scale.
 * String type
     - `StringType`: Represents character string values.
 * Binary type