Fixes issue when building documentation with spaces in the project directory path.

If you cloned GPUImage to a directory with spaces, then the docu build script won't work without quotes around the ${SOURCE_ROOT} path. This commit adds those quotes.

The error you get when you do have quotes is:
```
ERROR: AppledocException: Path or file '/Users/lm/Dropbox/Projects/Open' doesn't exist!
appledoc version: 2.1 (build 840)
```

In this case I've cloned the project into "/Users/lm/Dropbox/Projects/Open Source/GPUImage"