fix printing of DEStats

[ranocha]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Before this PR:

julia> SciMLBase.DEStats()
SciMLBase.DEStats
Number of function 1 evaluations:                  -1
Number of function 2 evaluations:                  -1
Number of W matrix evaluations:                    -1
Number of linear solves:                           -1
Number of Jacobians created:                       -1
Number of nonlinear solver iterations:             -1
Number of nonlinear solver convergence failures:   -1
Number of fixed-point solver iterations:                     -1
Number of fixed-point solver convergence failures:           -1
Number of rootfind condition calls:                -1
Number of accepted steps:                          -1
Number of rejected steps:                          -1

With this PR:

julia> SciMLBase.DEStats()
SciMLBase.DEStats
Number of function 1 evaluations:                  -1
Number of function 2 evaluations:                  -1
Number of W matrix evaluations:                    -1
Number of linear solves:                           -1
Number of Jacobians created:                       -1
Number of nonlinear solver iterations:             -1
Number of nonlinear solver convergence failures:   -1
Number of fixed-point solver iterations:           -1
Number of fixed-point solver convergence failures: -1
Number of rootfind condition calls:                -1
Number of accepted steps:                          -1
Number of rejected steps:                          -1