pgsql: doc: Fix some markups for support functions of index AMs

doc: Fix some markups for support functions of index AMs

All the documentation of index AMs has been using <replaceable> for
local_relopts.  This is a structure, so <structname> is a much better
choice.

Alexander has found the inconsistency for btree, while I have spotted
the rest when applying the concept of consistency to the docs.

Author: Alexander Lakhin, Michael Paquier
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/20200822133022.GC24782@paquier.xyz

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/77c1537f512e6ac2513f8695c795dc94cbf207ee

Modified Files
--------------
doc/src/sgml/brin.sgml   | 2 +-
doc/src/sgml/btree.sgml  | 2 +-
doc/src/sgml/gin.sgml    | 2 +-
doc/src/sgml/gist.sgml   | 2 +-
doc/src/sgml/spgist.sgml | 2 +-
5 files changed, 5 insertions(+), 5 deletions(-)