st: estout table column labels

["Dimitriy V. Masterov" <dvmaster@gmail.com>]

I am using the code below to generate the following table using
user-written -estout- after posting the results of margins:

--------------------------------------------
                   Weight             MPG
                   weight             mpg
--------------------------------------------
1._at                0.47***        -0.06
              [0.21,0.73]    [-0.35,0.23]
2._at                0.08           -0.21
             [-0.44,0.61]    [-0.45,0.04]
3._at               -0.30**         -0.44***
             [-0.49,-0.11]    [-0.56,-0.33]
4._at               -0.64***        -0.25***
             [-0.86,-0.41]    [-0.33,-0.18]
5._at               -0.09           -0.13*
             [-0.23,0.04]    [-0.23,-0.02]
6._at               -0.00            0.16
             [-0.00,0.00]    [-0.06,0.38]
7._at               -0.02*           0.24**
             [-0.03,-0.00]     [0.06,0.41]
--------------------------------------------
N                      74              74
--------------------------------------------

I can't seem to get rid of the lowercase variable names in the second
row. Is that possible somehow? I have scoured the documentation for
estout/esttab, but unless I am missing something, I just cant't make
them go away.

Here's the code that generates the table:

#delimit;
estimates drop _all;
sysuse auto, clear;

foreach var of varlist mpg weight {;
    zscore `var', stub(norm_);
    replace `var' = norm_`var';
    drop norm_`var';
};

estimates drop _all;
probit foreign c.mpg##c.mpg c.weight##c.weight;

margins, dydx(weight) at(weight=(-3(1)3)) post;
eststo m1;

probit foreign c.mpg##c.mpg c.weight##c.weight;
margins, dydx(mpg) at(mpg=(-3(1)3)) post;
eststo m2;

esttab, cells(b(fmt(2) star) ci(fmt(2)par)) unstack mtitle("Weight"
"MPG") nonumbers collabels(none);
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