3-improving.md中的vertexPosition没有申明，4-directional-lighting.md中的Enhanced directional lighting中的代码块存在语法问题

[Pycnocline]

getting-started中可能存在两个问题：

1.在3-improving.md文件中的Back to the Fragment shader (again)部分存在以下代码：

float fogValue = vertexPosition < fogEnd ? smoothstep(fogStart, fogEnd, vertexPosition) : 1.0;

此代码中的vertexPosition没有在上下文中提及且没有声明，推测应该为vertexDistance

2.在4-directional-lighting.md中的Enhanced directional lighting部分存在以下代码：

float NdotL = max(0.2, dot(normal, normalize(shadowLightPosition));

此代码中缺少一个')'

[WGrav01]

Translated to English:

There are two possible problems in getting-started:

1. The following code exists in the Back to the Fragment shader (again) section of the 3-improving.md file:

float fogValue = vertexPosition < fogEnd ? smoothstep(fogStart, fogEnd, vertexPosition) : 1.0;

The vertexPosition in this code is not mentioned in the context and is not declared, presumably it should be vertexDistance.

2. The following code exists in the Enhanced directional lighting section of 4-directional-lighting.md:

float NdotL = max(0.2, dot(normal, normalize(shadowLightPosition));

This code is missing a single ')'.

[WGrav01]

The first error was fixed in PR #18, I'll open another PR to fix the second issue. (Fixed in #20)