Doc fixes for MERGE statement

Doc fixes for MERGE statement

In commit 3d895bc846f2 I introduced a bogus semicolon mid-statement by
careless cut-n-paste; move it.  This had already been reported by Justin
Pryzby.

Also, change the styling a bit by avoiding names in CamelCase.  This is
more consistent with the style we use elsewhere.

Backpatch to 15.

Author: Vitaly Burovoy <[email protected]>
Reviewed-by: Vik Fearing <[email protected]>
Discussion: https://postgr.es/m/9afe5766-5a61-7860-598c-136867fad065@gmail.com
Discussion: https://postgr.es/m/20220819133016[email protected]