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

Fix typo in README
Signed-off-by: Anatolij Gustschin <agust@denx.de>

README

@@ -78,7 +78,7 @@ any version you might be interested in. Ofifcial releases are also
 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/