docs/devel/build-system.rst: Correct typo in example code

One of the example meson.build fragments incorrectly quotes some
symbols as 'CONFIG_FOO`; the correct syntax here is 'CONFIG_FOO'.
(This isn't a rST formatting mistake because the example is displayed
literally; it's just the wrong kind of quote.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20210726142338.31872-3-peter.maydell@linaro.org

docs/devel/build-system.rst

@@ -235,11 +235,11 @@ Target-independent emulator sourcesets:
   symbol::
 
     # Some targets have CONFIG_ACPI, some don't, so this is not enough
-    softmmu_ss.add(when: 'CONFIG_ACPI`, if_true: files('acpi.c'),
+    softmmu_ss.add(when: 'CONFIG_ACPI', if_true: files('acpi.c'),
                                         if_false: files('acpi-stub.c'))
 
     # This is required as well:
-    softmmu_ss.add(when: 'CONFIG_ALL`, if_true: files('acpi-stub.c'))
+    softmmu_ss.add(when: 'CONFIG_ALL', if_true: files('acpi-stub.c'))
 
 Target-dependent emulator sourcesets:
   In the target-dependent set lives CPU emulation, some device emulation and