Fix arguments for setting pdfindex for toys in tutorial

[kcormi]

Fixing a small mistake in the parametric fit tutorial documentation about using RooMultiPDF

As pointed out on cms talk, the current documentation says to use

--setParameters pdfindex=i --freezeParameters pdfindex=j

when doing the fit, to test the fit using a different pdf than was used to generate the toys. But the second part is not a valid argument, it should only take the parameter name, not a value.