doc: qemu-mips build instructions

Correct the make commands for the defconfigs.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/board/emulation/qemu-mips.rst b/doc/board/emulation/qemu-mips.rst
index f206039..d359251 100644
--- a/doc/board/emulation/qemu-mips.rst
+++ b/doc/board/emulation/qemu-mips.rst
@@ -29,28 +29,28 @@
 
 .. code-block:: bash
 
-   make qemu_mips
+   make qemu_mips_defconfig
    qemu-system-mips -M mips -bios u-boot.bin -nographic
 
 32 bit, little endian
 
 .. code-block:: bash
 
-   make qemu_mipsel
+   make qemu_mipsel_defconfig
    qemu-system-mipsel -M mips -bios u-boot.bin -nographic
 
 64 bit, big endian
 
 .. code-block:: bash
 
-   make qemu_mips64
+   make qemu_mips64_defconfig
    qemu-system-mips64 -cpu MIPS64R2-generic -M mips -bios u-boot.bin -nographic
 
 64 bit, little endian
 
 .. code-block:: bash
 
-   make qemu_mips64el
+   make qemu_mips64el_defconfig
    qemu-system-mips64el -cpu MIPS64R2-generic -M mips -bios u-boot.bin -nographic
 
 or using u-boot.bin from emulated flash: