Fix anchor links

[Xpirix]

Fix for #532

• Fixed the links on the Download QGIS for your platform, the Road Map and the Security pages
• Replace OSGEO4W with OSGeo4W

In the Road Map page, macOS is listed in the prereleases / nightly builds table at https://qgis.org/resources/roadmap/#qgis-prereleases. However, I couldn't find any link to prereleases / nightly builds for macOS (do they exist?) so I suggest hiding it for now. Any suggestions on that would be welcome.

On the security page, the text description at https://qgis.org/resources/support/security/#what-to-do-if-you-think-you-have-found-a-security-issue is a duplicate of the text at https://qgis.org/resources/support/#security-vulnerability-reporting.

So instead of:

If you believe you have found a security issue, such as vulnerabilities in QGIS or its dependencies, please refer to the security page for detailed information on how to report it responsibly. Before this, please do the following:

I suggest the following text on the actual security page (https://qgis.org/resources/support/security/):

If you believe you have found a security issue, such as vulnerabilities in QGIS or its dependencies, please do the following: