Fix the inconsistent prepare metadata value in checkpoint

Haribabu Kommi in WT-5984 a couple weeks ago you added prepare:# to the checkpoint metadata. It is inconsistent with all other configuration strings that we use or store in the metadata.

Here's an example:

...newest_stop_durable_ts=0,newest_stop_ts=-1,newest_stop_txn=-11,prepare:0,write_gen=...

Is there some reason you used a colon : instead of an equal =?

It was part of this commit. https://github.com/wiredtiger/wiredtiger/commit/a20a70a9ea2fc4cff0b6795e905d31d942b1ff5a