docs: fix spacing around "if not exists" brackets

Reported-by: Fabrízio de Royes Mello

Discussion: https://postgr.es/m/CAFcNs+qDD+QKcF8YCPQnjAxoWN61qY_YdFLB3iQqbWCLSCyY0g@mail.gmail.com

Author: Fabrízio de Royes Mello

doc/src/sgml/ref/create_server.sgml

@@ -21,7 +21,7 @@ PostgreSQL documentation
 
  <refsynopsisdiv>
 <synopsis>
-CREATE SERVER [IF NOT EXISTS] <replaceable class="parameter">server_name</replaceable> [ TYPE '<replaceable class="parameter">server_type</replaceable>' ] [ VERSION '<replaceable class="parameter">server_version</replaceable>' ]
+CREATE SERVER [ IF NOT EXISTS ] <replaceable class="parameter">server_name</replaceable> [ TYPE '<replaceable class="parameter">server_type</replaceable>' ] [ VERSION '<replaceable class="parameter">server_version</replaceable>' ]
     FOREIGN DATA WRAPPER <replaceable class="parameter">fdw_name</replaceable>
     [ OPTIONS ( <replaceable class="parameter">option</replaceable> '<replaceable class="parameter">value</replaceable>' [, ... ] ) ]
 </synopsis>

doc/src/sgml/ref/create_user_mapping.sgml

@@ -21,7 +21,7 @@ PostgreSQL documentation
 
  <refsynopsisdiv>
 <synopsis>
-CREATE USER MAPPING [IF NOT EXISTS] FOR { <replaceable class="parameter">user_name</replaceable> | USER | CURRENT_USER | PUBLIC }
+CREATE USER MAPPING [ IF NOT EXISTS ] FOR { <replaceable class="parameter">user_name</replaceable> | USER | CURRENT_USER | PUBLIC }
     SERVER <replaceable class="parameter">server_name</replaceable>
     [ OPTIONS ( <replaceable class="parameter">option</replaceable> '<replaceable class="parameter">value</replaceable>' [ , ... ] ) ]
 </synopsis>