[ci skip] Update PG adapter documentation

Per discussion in pull request #26622:

"Let's change it to PG::Error. The more specific classes mentioned are
subclasses, and the fact the raised exception is a PG::UndefinedColumn
doesn't change the fact that it's also a PG::Error." - matthewd

activerecord/lib/active_record/connection_adapters/postgresql/database_statements.rb

@@ -90,7 +90,7 @@ def query(sql, name = nil) #:nodoc:
         end
 
         # Executes an SQL statement, returning a PG::Result object on success
-        # or raising a PG exception otherwise.
+        # or raising a PG::Error exception otherwise.
         # Note: the PG::Result object is manually memory managed; if you don't
         # need it specifically, you may want consider the <tt>exec_query</tt> wrapper.
         def execute(sql, name = nil)