Improve grammar of message about two-phase state files.

When there's only one two-phase state file, there's also only one
long-running prepared transaction.  Adjust the message text
accordingly.

Nikhil Sontakke

Discussion: http://postgr.es/m/CAMGcDxcmR_DWZXXndGoPzVQx=B17A5=RviEA1qNaF=FWLy5Whw@mail.gmail.com