Fix GitHub search query example

[jahson]

This commit replaces outdated search query example with new working one.

[thisgeek]

This change is correct, but perhaps some citations are in order to get it approved by the maintainers.

Note: The extension qualifier is still supported by the code search API, according to the GitHub Docs Searching code (legacy) article, but it is not usable in the search on GitHub's webapp. See "Using Qualifiers" section of Understanding GitHub Code Search syntax for the list of currently supported qualifiers.