doc: cmd: bootmeth: Fix extlinunx -> extlinux typo

Fix a trivial typo in the bootmeth documentation.

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/cmd/bootmeth.rst b/doc/usage/cmd/bootmeth.rst
index 2903977..bac9fdf 100644
--- a/doc/usage/cmd/bootmeth.rst
+++ b/doc/usage/cmd/bootmeth.rst
@@ -48,7 +48,7 @@
 =====  ===  ==================  =================================
 Order  Seq  Name                Description
 =====  ===  ==================  =================================
-    0    0  extlinunx           Extlinux boot from a block device
+    0    0  extlinux            Extlinux boot from a block device
     1    1  efi                 EFI boot from an .efi file
     2    2  pxe                 PXE boot from a network device
     3    3  sandbox             Sandbox boot for testing