Fixed wrong sign in docu of ECC Extended Key Generation, Fixes #628

doc/crypt.tex

@@ -5185,7 +5185,7 @@ typedef struct {
 } ltc_ecc_curve;
 \end{verbatim}
 
-The curve must be of the form $y^2 = x^3 - a \cdot x + b$, and all of the \textit{const char*} parameters have to be encoded in hexadecimal format.
+The curve must be of the form $y^2 = x^3 + a \cdot x + b$, and all of the \textit{const char*} parameters have to be encoded in hexadecimal format.
 
 Advanced example of creating an ECC key:
 \begin{small}