docs: Correct wording about tilde range comparisons

[mloskot]

The "major level changes when the minor number is missing" was incorrect.

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This variant of the sentence is ambiguous, possibly incorrect, especially the second part of it:

semver/README.md

Lines 187 to 188 in 1558ca3

	The tilde (`~`) comparison operator is for patch level ranges when a minor
	version is specified and major level changes when the minor number is missing.

The tests also confirm it is not about major level changes but minor:

semver/constraints_test.go

Lines 383 to 388 in 1558ca3

	{"~1", "2.1.1", false},
	{"~1", "1.3.5", true},
	{"~1", "1.4", true},
	{"~1.x", "2.1.1", false},
	{"~1.x", "1.3.5", true},
	{"~1.x", "1.4", true},

Unless, I'm completely confused myself :)