Fix some typos (#1237)

Author: csuastt

docs/demos/pinn_forward.rst

@@ -33,6 +33,7 @@ Time-independent PDEs
    pinn_forward/helmholtz.2d.dirichlet.hpo
    pinn_forward/helmholtz.2d.neumann.hole
    pinn_forward/helmholtz.2d.sound.hard.abc
+   pinn_forward/Kovasznay.flow
 
 Time-dependent PDEs
 -------------------
@@ -50,7 +51,6 @@ Time-dependent PDEs
    pinn_forward/burgers.rar
    pinn_forward/allen.cahn
    pinn_forward/klein.gordon
-   pinn_forward/Kovasznay.flow
 
 - `Beltrami flow <https://github.com/lululxvi/deepxde/blob/master/examples/pinn_forward/Beltrami_flow.py>`_
 - `Wave propagation with spatio-temporal multi-scale Fourier feature architecture <https://github.com/lululxvi/deepxde/blob/master/examples/pinn_forward/wave_1d.py>`_
@@ -62,10 +62,10 @@ Integro-differential equations
 - `Integro-differential equation <https://github.com/lululxvi/deepxde/blob/master/examples/pinn_forward/ide.py>`_
 - `Volterra IDE <https://github.com/lululxvi/deepxde/blob/master/examples/pinn_forward/Volterra_IDE.py>`_
 
-fractional PDEs
+Fractional PDEs
 ---------------
 
-- `fractional Poisson equation in 1D <https://github.com/lululxvi/deepxde/blob/master/examples/pinn_forward/fractional_Poisson_1d.py>`_
-- `fractional Poisson equation in 2D <https://github.com/lululxvi/deepxde/blob/master/examples/pinn_forward/fractional_Poisson_2d.py>`_
-- `fractional Poisson equation in 3D <https://github.com/lululxvi/deepxde/blob/master/examples/pinn_forward/fractional_Poisson_3d.py>`_
-- `fractional diffusion equation <https://github.com/lululxvi/deepxde/blob/master/examples/pinn_forward/fractional_diffusion_1d.py>`_
+- `Fractional Poisson equation in 1D <https://github.com/lululxvi/deepxde/blob/master/examples/pinn_forward/fractional_Poisson_1d.py>`_
+- `Fractional Poisson equation in 2D <https://github.com/lululxvi/deepxde/blob/master/examples/pinn_forward/fractional_Poisson_2d.py>`_
+- `Fractional Poisson equation in 3D <https://github.com/lululxvi/deepxde/blob/master/examples/pinn_forward/fractional_Poisson_3d.py>`_
+- `Fractional diffusion equation <https://github.com/lululxvi/deepxde/blob/master/examples/pinn_forward/fractional_diffusion_1d.py>`_