settings.toml (environment) documentation had doubled backslashes

[dhalbert]

• Fixes settings.toml, CIRCUITPY_WIFI_SSID, and Unicode #11118 (@pgrady)

environment.rst, for settings.toml had doubled backslashes inside the sample TOML settings. But RST doesn't do backslash escape processing inside a code-block, so the doubled blackslashes were preserved

FRENCH2="\\u0153uvre" # same unicode string, using a 16-bit escape code

was fixed to be

FRENCH2="\u0153uvre" # same unicode string, using a 16-bit escape code

Testing: I examined the generated make html output to make sure it looks right.

[tannewt]

Thank you!