Use the thread safe form of TH*::Fit

The version of TH*::Fit which uses a string to designate the function
to use is not thread safe. The string is used to lookup a global
instance of the TF1 and that instance can be used simultaneously
but multiple Fits to store their results. However, the version of
TH*::Fit which takes an explicit pointer to a TF1 is safe.
Fixed incorrect syntax for Fit function

This typo was added by the previous commit. The problem was not found
earlier since this file is never compiled by scram.
Merge pull request #5066 from Dr15Jones/useThreadSafeFit

Use the thread safe form of TH*::Fit