please fix VERSION sample in the get infer page of the document

This isssue is document only.

In https://fbinfer.com/docs/getting-started#get-infer , the document says:
```
VERSION=0.XX.Y; \
curl -sSL "https://github.com/facebook/infer/releases/download/v$VERSION/infer-linux64-v$VERSION.tar.xz" \
| sudo tar -C /opt -xJ && \
sudo ln -s "/opt/infer-linux64-v$VERSION/bin/infer" /usr/local/bin/infer
```

But now the infer project have released 1.8.0, its version number is larger than 0.XX.Y.
I think that X.Y.Z is better than 0.XX.Y for version sample code.

```
VERSION=X.Y.Z; \
curl -sSL "https://github.com/facebook/infer/releases/download/v$VERSION/infer-linux64-v$VERSION.tar.xz" \
| sudo tar -C /opt -xJ && \
sudo ln -s "/opt/infer-linux64-v$VERSION/bin/infer" /usr/local/bin/infer
```