[math] xyexpo does not work in TFormula

[dpiparo]

Check duplicate issues.

• Checked for duplicates

Description

This was noticed in #20719.
The problem is that the user friendly name xyexpo is not correctly parsed by TFormula. Moreover, the formula stored for xyexpo is wrong.

Reproducer

Just do `TFormula("f","xyexpo").

ROOT version

all

Installation method

all

Operating system

all

Additional context

No response

[ferdymercury]

This one also fails:

auto f2 = new TF2("xylandaun", "xylandaun", -1, 1, -1, 1);

with message

Error in <prepareMethod>: Can't compile function TFormula____id5579575194643800961 prototype with arguments Double_t*,Double_t*
Error in <TFormula::InputFormulaIntoCling>: Error compiling formula expression in Cling
Error in <TFormula::ProcessFormula>: "n" has not been matched in the formula expression
Error in <TFormula::ProcessFormula>: Formula "[p0]*TMath::Landau(x,[p1],[p2],false)*TMath::Landau(y,[p3],[p4],false)n" is invalid !