config/core.txt: fix minor issues for `core.sparseCheckoutCone`

config/core.txt: fix minor issues for `core.sparseCheckoutCone`

The sparse checkout feature can be used in "cone mode" or "non-cone
mode". In this one instance in the documentation, we refer to the latter
as "non cone mode" with whitespace rather than a hyphen. Align this with
the rest of our documentation.

A few words later in the same paragraph, there's mention of "a more
flexible patterns". Drop that leading "a" to fix the grammar.

Signed-off-by: Martin Ågren <redacted>
Acked-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>