best-practices: reword high-degree-of-differentiation to high selectivity

[yahonda]

What is changed, added or deleted? (Required)

best-practices/tidb-best-practices.md's index-creation guidance used a one-off phrase "columns with a high degree of differentiation" for what every other place in the corpus calls "selectivity" (see statistics.md, system-variables.md). Rewording to the established term for consistency; the Japanese translation will be updated to match (選択性が高い列) in a follow-up PR.

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