Fix code comment in postgres_fdw.c

Hi Hackers,

This Koshi Shibagaki.
I found out that there is a mistake written in contrib/postgres_fdw/postgres_fdw.c.

Patch file is attached.

The non-existent function name " ExecCheckRTEPerms " was written in 
the comment in postgresBeginForeignScan.
This mistake is considered to have occurred at commit ID: a61b1f74.
The function name was changed to "ExecCheckPermissions" and comments 
related to this change was fixed, 
however only the comment in postgresBeginForeignScan was not fixed.

Best

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Fujitsu Limited
Koshi Shibagaki
shibagaki.koshi@fujitsu.com