doc: Fix indefinite article an -> a before nouns that begin with 'h'

Although the rule also allows 'an' for historical reasons,
'a' is more common because 'h' is pronounced.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/qemu-doc.texi b/qemu-doc.texi
index 0af0ff4..84dad19 100644
--- a/qemu-doc.texi
+++ b/qemu-doc.texi
@@ -78,7 +78,7 @@
 
 @end itemize
 
-QEMU can run without an host kernel driver and yet gives acceptable
+QEMU can run without a host kernel driver and yet gives acceptable
 performance.
 
 For system emulation, the following hardware targets are supported: