fix typo

Author: WeiqunZhang

README.md

@@ -56,7 +56,7 @@ Local automatic variables can be defined in the expression.  For example,
 
 ```c++
    Parser parser("r2=x*x+y*y; r=sqrt(r2); cos(a+r2)*log(r)"
-   parser.setConstant(a, ...);
+   parser.setConstant("a", ...);
    parser.registerVariables({"x","y"});
    auto f = parser.compile<2>();  // 2 because there are two variables.
 ```