Fix script name in README.

The script was rewritten in Perl, and renamed from regress.sh to regress.pl,
back in 2012.

src/interfaces/libpq/test/README

 This is a testsuite for testing libpq URI connection string syntax.
 
 To run the suite, use 'make installcheck' command.  It works by
-running 'regress.sh' from this directory with appropriate environment
+running 'regress.pl' from this directory with appropriate environment
 set up, which in turn feeds up lines from 'regress.in' to
 'uri-regress' test program and compares the output against the correct
 one in 'expected.out' file.