README: update documentation (availability, links, etc.)

Fix typo in README

Signed-off-by: Anatolij Gustschin <agust@denx.de>
diff --git a/README b/README
index b3423e3..5cbe7c1 100644
--- a/README
+++ b/README
@@ -78,7 +78,7 @@
 available for FTP download from the ftp://ftp.denx.de/pub/u-boot/
 directory.
 
-Pre-build (and tested) images are available from
+Pre-built (and tested) images are available from
 ftp://ftp.denx.de/pub/u-boot/images/