qapi: Fix qcow2 encryption doc typo

Present since commit b25b387f (2.10).
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Message-Id: <20190206202848.16999-1-eblake@redhat.com>
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>

qapi/block-core.json

@@ -2998,7 +2998,7 @@
 
 ##
 # @BlockdevQcow2EncryptionFormat:
-# @aes: AES-CBC with plain64 initialization venctors
+# @aes: AES-CBC with plain64 initialization vectors
 #
 # Since: 2.10
 ##