doc: fix typo commnad

fix typo from "commnad" to "command"

Signed-off-by: Aristo Chen <aristo.chen@canonical.com>
diff --git a/doc/usage/cmd/part.rst b/doc/usage/cmd/part.rst
index e7faecc..e7f6e54 100644
--- a/doc/usage/cmd/part.rst
+++ b/doc/usage/cmd/part.rst
@@ -50,7 +50,7 @@
     varname
         an optional environment variable to store the list of partitions value into.
 
-The 'part start' commnad sets an environment variable to the start of the partition (in blocks),
+The 'part start' command sets an environment variable to the start of the partition (in blocks),
 part can be either partition number or partition name.
 
     interface