cmd/bootefi: correct command syntax

For indicating the address and size of a memory region other commands use a
<addr>[:<size>] syntax. Do the same for bootefi.

Fixes: 2058983689 ("cmd: bootefi: restore ability to boot arbitrary blob")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

doc/usage/cmd/bootefi.rst

@@ -9,10 +9,10 @@ Synopsis
 
 ::
 
-    bootefi [image_addr] [fdt_addr [image_size]]
-    bootefi bootmgr [fdt_addr]
-    bootefi hello [fdt_addr]
-    bootefi selftest [fdt_addr]
+    bootefi <image_addr>[:<image_size>] [<fdt_addr>]
+    bootefi bootmgr [<fdt_addr>]
+    bootefi hello [<fdt_addr>]
+    bootefi selftest [<fdt_addr>]
 
 Description
 -----------