fixed typo in section 1.6

[Flyagin]

• [x] I provide my work under the project license.
• [x] I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.

Changes

Fixed typo in section 1.6: “values” should be replaced with “variables”.

Context

Consider the beginning of this sentence: “Since Git might read the same configuration variable value from more than one file, … ” — having read this part of the sentence, one concludes that a “value” belongs to a “variable”. But the remaining part of the sentence implies that a “value” belongs to a value (“one of these values”) (“ … it's possible that you have an unexpected value for one of these values and you don't know why.”)