docs: Fix typo in the default name of the qemu-system-x86_64 binary

It's a '-' between 'qemu' and 'system', not a '_'.

Fixes: 324b2298fe ("docs/system: convert Texinfo documentation to rST")
Message-Id: <20210401051829.3169438-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>

docs/defs.rst.inc

@@ -10,6 +10,6 @@
    incorrectly in boldface.
 
 .. |qemu_system| replace:: qemu-system-x86_64
-.. |qemu_system_x86| replace:: qemu_system-x86_64
+.. |qemu_system_x86| replace:: qemu-system-x86_64
 .. |I2C| replace:: I\ :sup:`2`\ C
 .. |I2S| replace:: I\ :sup:`2`\ S