Corrected a minor bug in the arima doc

Author: Qian, Hai

src/ports/postgres/modules/tsa/arima.sql_in

@@ -41,7 +41,7 @@ respectively.
 
 The ARIMA training function has the following syntax.
 <pre class="syntax">
-arima( input_table,
+arima_train( input_table,
        output_table,
        timestamp_column,
        timeseries_column,
@@ -233,7 +233,7 @@ The output table produced by the forecast function contains the following column
 @examp
 -# View online help for the ARIMA training function.
 <pre class="example">
-SELECT madlib.arima();
+SELECT madlib.arima_train();
 </pre>
 
 -#  Create an input data set.