  The asymptotic behavior of Wilson loops in the large-size limit
($L\rightarrow\infty$) in confining gauge theories with area law is controlled
by effective string theory (EST). The $L^{-2}$ term of the large-size expansion
for the logarithm of Wilson loop appears within EST as a two-loop correction.
Ultraviolet divergences of this two-loop correction for polygonal contours can
be renormalized using an analytical regularization constructed in terms of
Schwarz-Christoffel mapping. In the case of triangular Wilson loops this method
leads to a simple final expression for the $L^{-2}$ term.
