Fix typo in pg_dump's support for dumping collations from pre-v10 servers.

Dunno what 'p' was supposed to mean, but since neither the code below
here nor pg_collation.h think it's valid, it must be a mistake.

Per report from Thomas Kellerer.

Discussion: https://postgr.es/m/og9q8f%24oes%241%40blaine.gmane.org

src/bin/pg_dump/pg_dump.c

@@ -13039,7 +13039,7 @@ dumpCollation(Archive *fout, CollInfo *collinfo)
 						  collinfo->dobj.catId.oid);
 	else
 		appendPQExpBuffer(query, "SELECT "
-						  "'p'::char AS collprovider, "
+						  "'c'::char AS collprovider, "
 						  "collcollate, "
 						  "collctype, "
 						  "NULL AS collversion "