MeasurementMention fails on "fourty"

[kwalcock]

number.yml has "fourty" instead of "forty". This eventually leads to an exception being thrown in MeasurementMention.neNorm when "fourty" is encountered. Correcting the typo fixes the immediate problem.

However, this is also another case of needing to throw the exception upon Mention creation rather than later when something queries for some information about the Mention, in this case, neNorm.

Example sentences are

(c) a retention licence, fourty five days; and .
(c) an exploration licence, fourty days; .
(d) a retention licence, fourty five days; and .