[PATCH] DOCS: Fix inconsistent text description punctuation in UPDATE FOR PORTION OF

Hi,
I noticed an inconsistency in the documentation between UPDATE and DELETE 
commands regarding the FOR PORTION OF clause description. 

In doc/src/sgml/ref/update.sgml (line 169), the description reads:
  "must be in the form ( portion ) where portion is an expression..." 

In doc/src/sgml/ref/delete.sgml (line 166), it reads:
 "must be in the form ( portion ), where portion is an expression..." 

The UPDATE documentation misses a comma before "where" that DELETE has.
For consistency, add the comma to the UPDATE documentation to match DELETE.

I have recompiled the documentation and verified that
share/doc/postgresql/html/sql-update.html and
share/doc/postgresql/html/sql-delete.html are now consistent.

Best regards,
Zizhuan Liu