doc/opal-flash: fix typo

Fixes: #85
Suggested-by: Joel Nider <JOELN@il.ibm.com>
Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>

doc/opal-api/opal-flash-110-111-112.rst

@@ -8,7 +8,7 @@ There are three OPAL calls for interacting with flash devices: ::
  #define OPAL_FLASH_ERASE	112
 
 Multiple flash devices are supported by OPAL - each of these calls takes an id
-parameter, which much match an ID found in the corresponding ``ibm,opal/flash@n``
+parameter, which must match an ID found in the corresponding ``ibm,opal/flash@n``
 device tree node. See :ref:`device-tree/ibm,opal/flash` for details of
 the device tree bindings.