Fix typo: commmand -> command.

Signed-off-by: Vagrant Cascadian <vagrant@debian.org>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/doc/README.update b/doc/README.update
index eab124c..d37f2c4 100644
--- a/doc/README.update
+++ b/doc/README.update
@@ -51,7 +51,7 @@
 to be prepared. Refer to the doc/uImage.FIT/ directory for more details on FIT
 images.
 
-This mechanism can be also triggered by the commmand "fitupd".
+This mechanism can be also triggered by the command "fitupd".
 If an optional, non-zero address is provided as argument, the TFTP transfer
 is skipped and the image at this address is used.
 The fitupd command is enabled by CONFIG_CMD_FITUPD.