doc: Bring in FIT signature files

Bring these files into the documentation.

Fix 'wtih' and 'it' typos and repeated 'could' while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/cmd/source.rst b/doc/usage/cmd/source.rst
index 61a4505..6f5fa28 100644
--- a/doc/usage/cmd/source.rst
+++ b/doc/usage/cmd/source.rst
@@ -22,7 +22,7 @@
 * Flat Image Tree (FIT)
 
 The benefit of the FIT images is that they can be signed and verifed as
-decribed in :download:`signature.txt <../../uImage.FIT/signature.txt>`.
+described in :doc:`../fit/signature`.
 
 Both formats can be created with the mkimage tool.