Incorrect types table for AVG in docs

The docs for AVG state that AVG for int & long produces a long, but the source for those classes actually produce a double.

• https://github.com/apache/pig/blob/trunk/src/org/apache/pig/builtin/AVG.java
• https://github.com/apache/pig/blob/trunk/src/org/apache/pig/builtin/LongAvg.java
• https://github.com/apache/pig/blob/trunk/src/org/apache/pig/builtin/IntAvg.java