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

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>