fix readme [skip ci]

README.md

@@ -36,7 +36,9 @@ This package is extracted from [AstroNbodySim.jl](https://github.com/JuliaAstroS
 ### Differencing
 
 Central differencing scheme is defined as
+
 $$\left(\frac{\partial u}{\partial x}\right)_{i, j}=\frac{1}{2 \Delta x}\left(u_{i+1, j}-u_{i-1, j}\right)+O(\Delta x)$$
+
 Suppose we have an 1D data, and $\delta x = 5$:
 ```julia
 julia> d1 = [1,2,1]
@@ -76,11 +78,16 @@ julia> grad_central(1, 1, 1, d3)
 ### FDM solver
 
 #### Poisson equation
-$$\delta u = f$$
+$$\Delta u = f$$
+
 can be discretized to
+
 $$\frac{u_{i+1}-2 u_{i}+u_{i-1}}{\Delta x^{2}}=f_{i}, i = 1, 2, \cdots, N$$
+
 In Dirichlet boundary conditions,
-$$\frac{1}{\Delta x^2} \begin{pmatrix}
+
+$$
+\frac{1}{\Delta x^2} \begin{pmatrix}
         -2 &      1 &      0 & \cdots & \cdots &      0 \\
         1 &     -2 &      1 &      0 & \cdots & \vdots \\
         0 &      1 &     -2 &      1 & \cdots & \vdots \\
@@ -94,9 +101,11 @@ $$\frac{1}{\Delta x^2} \begin{pmatrix}
 \end{pmatrix}
 = \begin{pmatrix}
     f_1 \\ f_2 \\ \vdots \\ \vdots \\ \vdots \\ f_{N-1} \\ f_N
-\end{pmatrix}$$
+\end{pmatrix}
+$$
 
 The Poisson problem is converted to solving a matrix equation
+
 $$\mathbf{A} \cdot \mathbf{u} = \mathbf{f}$$
 
 `PhysicalFDM.jl` is here to provide user-friendly tools to generate `\mathbf{A}` matrix.