[samuelson] Inconsistent usage of ($a$, $b$) and ($\alpha$, $\beta$)

[HumphreyYang]

In the lecture, the usage of $\alpha$ and $a$, $\beta$ and $b$ are inconsistent.

For instance, in the class Samuelson, we used

    α : scalar
        Marginal propensity to consume
    β : scalar
        Accelerator coefficient

but in the main text, we defined:

- $a$ is the marginal propensity to consume in the Keynesian
  consumption function $C_t = a Y_{t-1} + \gamma$.
- $b$ is the "accelerator coefficient" in the "investment
  accelerator" $I_t = b (Y_{t-1} - Y_{t-2})$.

This is being fixed in the Chinese version.