man page: Fix rst2man warning
man page: Fix rst2man warning

Introduced in 378d3f6. The warning was:

    <stdin>:121: (ERROR/3) Unknown target name: "standard key names<https://stb-tester.com/stb-tester-one/rev2015.1/getting-started#remote-control-key-names>".

This used to render like this in the terminal:

    stbt provides some sensible default mappings when there is an
    obvious match for our
    `standard key names<https://stb-tester.com/stb-tester-one/rev2015.1/getting-started#remote-control-key-names>`_
    .

Now it renders like this in the terminal (with "standard key names"
underlined but not hyperlinked):

    stbt provides some sensible default mappings when there is an
    obvious match for our standard key names.

The HTML version will have a hyperlink as expected.

stb-tester/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester