glad -h help for API argument
The documentation that the argparse module gives for the API is a little bit misleading.

I'm using the following version of glad:
```
$pip3 show glad
Name: glad
Version: 0.1.28
```
Argparse shows the following doc:
```
  --api API             API type/version pairs, like "gl=3.2,gles=", no
                        version means latest
```
However, when I use that in the following example:

```
glad    --omit-khrplatform      \
        --local-files           \
        --profile core          \
        --generator c           \
        --extensions /dev/null  \
        --api gl=3.3,gles=2.0  \
        --out-path .            \
        --spec gl
```
This raises the following error:
```
ValueError: Unknown API "gles" for specification "gl"
```
The however, if I use --api gl=3.3,gles**2**, glad does it's job successfully. I'm quite new to opengl, and perhaps this is very clear to someone with more opengl experience than I have. For me as newcomer to opengl/glad its very nice when I can use the examples from argparse, without ValueErrors

Best regards,