Better build instructions.

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpasyncclient/trunk@1749365 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/BUILDING.txt b/BUILDING.txt
index a4c4cd5..cc22570 100644
--- a/BUILDING.txt
+++ b/BUILDING.txt
@@ -46,6 +46,5 @@
 
 Execute the following commands in order to generate Javadoc:
 
-mvn install
-mvn javadoc:aggregate
+mvn compile javadoc:aggregate