Fix maximum page size at eviction to maximum page size seen at eviction

[schwarmco]

In this commit mongodb/mongo@838d364 the wording of this property on wiredtiger.cache changed. This PR adds the seen to the docs as it is the current wording:

test> db.version()
7.0.14
test> db.runCommand( { serverStatus: 1 } ).wiredTiger.cache['maximum page size at eviction']

test> db.runCommand( { serverStatus: 1 } ).wiredTiger.cache['maximum page size seen at eviction']
0