Add small detail to RAISE statement descripton

Hi,

There is a slight syntax inaccuracy in the description of the RAISE
statement - assignments of parameter values in USING.
(Chapter 43. PL/pgSQL — SQL Procedural Language / 43.9 Errors and
Messages / 43.9.1 Reporting Errors and Messages):

"...You can attach additional information to the error report by
writing USING followed by option = expression items..."
It should, apparently, be like this: "...option { = | := } expression..."

The patch corrects this little omission. Attached: fix_doc_raise.patch

Regards, Igor Gnatyuk