[doc] fix a potential grammer mistake

[Junwang Zhao]

I think in the following sentence, were should be replaced with have,
what do you think?

```
                        /*
-                        * We were just issued a SAVEPOINT inside a
transaction block.
+                        * We have just issued a SAVEPOINT inside a
transaction block.
                         * Start a subtransaction.  (DefineSavepoint already did
                         * PushTransaction, so as to have someplace to
put the SUBBEGIN
                         * state.)
```


-- 
Regards
Junwang Zhao

0001-doc-fix-a-potential-grammer-mistake.patch
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