RISC-V: Kconfig: Fix help text of CMDLINE_EXTEND

It is the built-in command line appended to the bootloader command line,
not the bootloader command line appended to the built-in command line.

Fixes: 3aed8c4 ("RISC-V: Update Kconfig to better handle CMDLINE")
Signed-off-by: 谢致邦 (XIE Zhibang) <Yeking@Red54.com>
Link: https://lore.kernel.org/r/tencent_A93C7FB46BFD20054AD2FEF4645913FF550A@qq.com
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>

Author: Red54

arch/riscv/Kconfig

@@ -1176,8 +1176,8 @@ config CMDLINE_FALLBACK
 config CMDLINE_EXTEND
 	bool "Extend bootloader kernel arguments"
 	help
-	  The command-line arguments provided during boot will be
-	  appended to the built-in command line. This is useful in
+	  The built-in command line will be appended to the command-
+	  line arguments provided during boot. This is useful in
 	  cases where the provided arguments are insufficient and
 	  you don't want to or cannot modify them.