DOC: correct typo in docstring

Correct the sign of the last term of the log likelihood expression.
Clarify the meaning of the (not standard) vertical bars notation.

Author: tartopohm

statsmodels/regression/linear_model.py

@@ -794,9 +794,10 @@ def loglike(self, params):
         -----
         .. math:: -\frac{n}{2}\log SSR
                   -\frac{n}{2}\left(1+\log\left(\frac{2\pi}{n}\right)\right)
-                  -\frac{1}{2}\log\left(\left|W\right|\right)
+                  +\frac{1}{2}\log\left(\left|W\right|\right)
 
-        where :math:`W` is a diagonal weight matrix matrix and
+        where :math:`W` is a diagonal weight matrix matrix,
+        :math:`\leftW\right|` is its determinant, and
         :math:`SSR=\left(Y-\hat{Y}\right)^\prime W \left(Y-\hat{Y}\right)` is
         the sum of the squared weighted residuals.
         """