Update index.md (#719)

Changed math tagging, that doesn't work, by $$ tagging, what it works and have the desired result

Author: Masacroso

docs/src/index.md

@@ -10,9 +10,7 @@ and for solving differential and integral equations.
 A basic approach of computational mathematics that ApproxFun exploits is expansion
 in a basis
 
-```math
-f(x) \approx \sum_{k=1}^n c_k \psi_k(x)
-```
+$$f(x) \approx \sum_{k=1}^n c_k \psi_k(x)$$
 
 Some traditional examples of bases $\psi_1(x),\psi_2(x),\ldots$ are
 1. Taylor series: $1,z,z^2,\ldots$