Documentation/util/kconfig: Remove silentoldconfig

The "silentoldconfig" target has been removed in Linux 5.13's kconfig
(CB:37152). As explained by Michal Marek at
https://lkml.org/lkml/2011/8/31/189, the "silentoldconfig" target has
become an internal interface and "oldconfig" is just as silent now.
Therefore, correct the target for syntax checking.

Change-Id: I8416bd4a96d15415f46c591ceb26ebb29aef1ab0
Signed-off-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56878
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Jakub Czapiga <jacz@semihalf.com>
diff --git a/Documentation/getting_started/kconfig.md b/Documentation/getting_started/kconfig.md
index 819eb72..fff186e 100644
--- a/Documentation/getting_started/kconfig.md
+++ b/Documentation/getting_started/kconfig.md
@@ -1188,7 +1188,7 @@
 ## Syntax Checking:
 
 The Kconfig utility does some basic syntax checking on the Kconfig tree.
-Running "make silentoldconfig" will show any errors that the Kconfig utility
+Running "make oldconfig" will show any errors that the Kconfig utility
 sees.
 
 ### util/kconfig_lint