Fix README: .env.local in test should not outrank .env.test.local

[hikmetba-bit]

Fixes #543.

The Customizing Rails section's example used Dotenv::Rails.files.unshift(".env.local"), which puts .env.local at index 0 — the highest precedence, ahead of .env.test.local.

Per the precedence table earlier in the same README, .env.local is meant to sit at the 2nd precedence tier in every environment (between the .env.<env>.local and .env.<env> tiers) — the same position it already has by default in development and production (it's just excluded by default in test, per lib/dotenv/rails.rb's (".env.local" unless env.test?)). The test-only snippet that re-adds it should insert it at that same position, not shove it in front of everything.

Verified the replacement snippet actually produces the right order:

files = [".env.test.local", ".env.test", ".env"]
index = files.index(".env.test.local")&.succ || files.index(".env.test") || 0
files.insert(index, ".env.local")
p files
# => [".env.test.local", ".env.local", ".env.test", ".env"]

Docs-only change (README example code), matching the fix the issue reporter already worked out and verified.

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