doc: fix file extension for flattened image tree blob

Different sections in the document suggest flattened image tree blob
files have a file name extension of .itb. Fix the list of file extensions
to reflect that.

Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>

doc/uImage.FIT/source_file_format.txt

@@ -56,7 +56,7 @@ FIT is formally a flattened device tree (in the libfdt meaning), which
 conforms to bindings defined in this document.
 
 .its	- image tree source
-.fit	- flattened image tree blob
+.itb	- flattened image tree blob
 
 c) Image building procedure