Correct QEMU case typo

Verified exhaustiveness thanks to:

```bash
grep -ri 'QEMU' | sed -E 's/qemu|QEMU|Qemu//g' | grep -i qemu
```

Signed-off-by: Benjamin Loison <benjamin.loison@orange.fr>

docs/drvqemu.rst

@@ -511,7 +511,7 @@ Import and export of libvirt domain XML configs
 
 The QEMU driver currently supports a single native config format known as
 ``qemu-argv``. The data for this format is expected to be a single line first a
-list of environment variables, then the QEMu binary name, finally followed by
+list of environment variables, then the QEMU binary name, finally followed by
 the QEMU command line arguments
 
 Converting from QEMU args to domain XML