dblink docs:  fix typo to use "connname" (3 n's), not "conname"
dblink docs:  fix typo to use "connname" (3 n's), not "conname"

This makes the parameter names match the documented prototype names.

Report by Erwin Brandstetter

Backpatch through 9.0

zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl