cosmetic: doc: uImage.FIT: fix typos

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>

doc/uImage.FIT/command_syntax_extensions.txt

@@ -170,7 +170,7 @@ bootm 200000
 
 In case of the new uImage argument syntax, the address portion of any argument
 can be omitted. If <addr3> is omitted, then it is assumed that image at
-<addr2> should be used. Similarly, when <addr2> is omitted, is is assumed that
+<addr2> should be used. Similarly, when <addr2> is omitted, it is assumed that
 image at <addr1> should be used. If <addr1> is omitted, it is assumed that the
 current image address is to be used. For example, consider the following
 commands:

doc/uImage.FIT/source_file_format.txt

@@ -62,7 +62,7 @@ c) Image building procedure
 The following picture shows how the new uImage is prepared. Input consists of
 image source file (.its) and a set of data files. Image is created with the
 help of standard U-boot mkimage tool which in turn uses dtc (device tree
-compiler) to produce image tree blob (.itb).  Resulting .itb file is is the
+compiler) to produce image tree blob (.itb).  Resulting .itb file is the
 actual binary of a new uImage.