Adjust down breakpoint offset to be 0.04

[BPScott]

WHY are these changes introduced?

Fixes #7237

WHAT is this pull request doing?

This ensures that $p-breakpoint-*-{down,only} breakpoint max-width values in ems are representable in 4 decimal places. This avoids rounding issues when using node-sass's default rounding precision of 5 decimal places, as prior to this change our media queries required a precision of 6 decimal places (e.g. $p-breakpoints-md-down: '(max-width: 47.996875em)'; is now $p-breakpoints-md-down: '(max-width: 47.9975em)';