Better build instructions.

git-svn-id: https://svn.apache.org/repos/asf/httpcomponents/httpasyncclient/branches/4.0.x@1749364 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/BUILDING.txt b/BUILDING.txt
index da5d0f7..46b4159 100644
--- a/BUILDING.txt
+++ b/BUILDING.txt
@@ -36,6 +36,5 @@
 
 Execute the following commands in order to generate Javadoc:
 
-mvn install
-mvn javadoc:aggregate
+mvn compile javadoc:aggregate