Docs: Standardize "cannot" usage in SGML source

Hi Hacker,

While reviewing a patch, I happened to notice the PG docs prefer “cannot”
than “can not”.

Under doc/src/sgml, if we count:
```
sgml % grep -r "cannot" . | wc -l
818
```

“Cannot” has 818 occurrences, but “can not” has only 3:
```
sgml % grep -rE "can\s+not" .
./ecpg.sgml: some SQL statement is declared, "database" can not be used
as a cursor
./func/func-aggregate.sgml: The <parameter>key</parameter> can not
be null. If the
./func/func-aggregate.sgml: The <parameter>key</parameter> can not
be null. If the
```

So, maybe we should fix these 3 and keep a consistent usage of “cannot”.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/