Docs: Corrected typo in the basics of PE page, maths section.

I corrected a small typo in the basics of PE page, maths section. The parenthesis containing the squared exponent should read (y_i - (t_i * m + c)) instead of (y_i - (t_i * x + c)) for a model H defined by y = m*x + c.

docs/basics-of-parameter-estimation.txt

@@ -41,7 +41,7 @@ Gaussian distributed as such:
 .. math::
 
    P(y_i, t_i| m, c, H) = \frac{1}{\sqrt{2\pi\sigma^2}}
-                         \mathrm{exp}\left(\frac{-(y_i - (t_i x + c))^2}{2\sigma^2}\right) \,.
+                         \mathrm{exp}\left(\frac{-(y_i - (t_i m + c))^2}{2\sigma^2}\right) \,.
 
 Next, we assume that all data points are independent. As such,