configure.ac: check for MPI_T.3 (not MPI_T.5) for pre-built docs

We moved the MPI_T man page from section 5 to section 3 (where it
belongs), but forgot to update the configure check that uses that man
page as a sentinel to know if the pre-built docs are available.

Thanks to Orion Poplawski for noticing the problem.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

Author: jsquyres

configure.ac

@@ -1070,7 +1070,7 @@ AS_IF([test -z "$LEX" || \
 # Setup HTML and man page processing
 #
 
-OAC_SETUP_SPHINX([$srcdir/docs/_build/man/MPI_T.5],
+OAC_SETUP_SPHINX([$srcdir/docs/_build/man/MPI_T.3],
                  dnl Note that we have to double escape the string below
                  dnl so that the # it contains doesn't confuse the Autotools
                  [[https://docs.open-mpi.org/en/main/developers/prerequisites.html#sphinx-and-therefore-python]])