Fix comment in psql's describe.c

Procedures are supported since v11 and \dfp can be used since this
version, but it was not mentioned as a supported option in the
description of describeFunctions() which handles \df in psql.

Extracted from a larger patch.

Author: Fabien Coelho
Discussion: https://postgr.es/m/alpine.DEB.2.21.1908281618520.28828@lancre

src/bin/psql/describe.c

@@ -307,6 +307,7 @@ describeTablespaces(const char *pattern, bool verbose)
  *
  * a for aggregates
  * n for normal
+ * p for procedure
  * t for trigger
  * w for window
  *