Spec for Fog type incorrect

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/191)) by Anonymous.**

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Hey guys,

I've recently been playing around with the fog setting and it seems like the SDF spec is incorrect. Right now, the types of fog are listed as linear, quadratic, and exponential, but I was getting an error. I looked at the gazebo source though for scenes and it looks like the Fog types from OGRE were just carried over:

```
#!c++

/// \brief Set the fog parameters.
/// \param[in] _type Type of fog: "linear", "exp", or "exp2".
/// \param[in] _color Color of the fog.
/// \param[in] _density Fog density.
/// \param[in] _start Distance from camera to start the fog.
/// \param[in] _end Distance from camera at which the fog is at max
/// density.
public: void SetFog(const std::string &_type,
const ignition::math::Color &_color,
const double _density, const double _start,
const double _end);
```
I think it would best to update the latest SDF spec to reflect this.