[PATCH] libpq: Fix incorrect message type in getBackendKeyData() comment

Hi all,  

This is a trivial fix for the comment of getBackendKeyData() function in fe-protocol3.c.  
The comment incorrectly stated that the BackendKeyData message type is 'v', when in fact it is 'K' as defined in protocol.h. This fix corrects the comment to match the actual message type used in the protocol.  
No functional changes are involved.  

Patch is attached below.  

Thanks,  
zourenli  
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