doc: correct heading level in printenv man-page

The 'Configuration' heading should be on level 2, not on level 1.

Fixes: eaa268589e9c ("doc: man-page for the printenv command")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/usage/cmd/printenv.rst b/doc/usage/cmd/printenv.rst
index 9cb20f6..d4184fd 100644
--- a/doc/usage/cmd/printenv.rst
+++ b/doc/usage/cmd/printenv.rst
@@ -78,7 +78,7 @@
     =>
 
 Configuration
-=============
+-------------
 
 UEFI variables are only supported if CONFIG_CMD_NVEDIT_EFI=y. The value of UEFI
 variables can only be displayed if CONFIG_HEXDUMP=y.