pgsql: doc: Fix man page whitespace issues

doc: Fix man page whitespace issues

Whitespace between tags is significant, and in some cases it creates
extra vertical space in man pages.  The fix is to remove some newlines
in the markup.

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/9cae39b8e6a53decb37cce22852bb2e6d0e3f5d3

Modified Files
--------------
doc/src/sgml/ref/cluster.sgml          | 1 -
doc/src/sgml/ref/create_extension.sgml | 3 +--
doc/src/sgml/ref/create_procedure.sgml | 3 +--
3 files changed, 2 insertions(+), 5 deletions(-)