docs: List default value for Environment::prefix_separator

[anoadragon453]

List the default value for the Environment::prefix_separator property.

The default for prefix_separator is determined by:

config-rs/src/env.rs

Lines 230 to 234 in ad773ab

	let prefix_separator = match (self.prefix_separator.as_deref(), self.separator.as_deref()) {
	(Some(pre), _) => pre,
	(None, Some(sep)) => sep,
	(None, None) => "_",
	};

[coveralls]

Pull Request Test Coverage Report for Build 15148653414

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Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 65.295%

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Totals
Change from base Build 14323491851:	0.0%
Covered Lines:	952
Relevant Lines:	1458

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