minor #7216 fix build command for Linux (snoek09)

This PR was submitted for the 3.1 branch but it was merged into the 2.7 branch instead (closes #7216).

Discussion
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fix build command for Linux

This was spotted by @sawmurai during the SymfonyCon Berlin hackday.

Can anyone check if the command for Windows is correct?

Commits
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7107b96 fix build command for Linux

Author: xabbuh

contributing/documentation/overview.rst

@@ -280,7 +280,8 @@ purposes following these steps:
    .. code-block:: terminal
 
        # Linux and macOS
-       $ ./_build/make html
+       $ cd _build/
+       $ make html
 
        # Windows
        $ _build\make html