Top navigation bar doesn't use Seldon's pink (#ff4072), and README uses wrong form of "lets"

[MarcoGorelli]

Admittedly these are very minor issues, but I thought they might be appreciated.

The first one is analogous to SeldonIO/alibi#69: the top navigation bar shows the default Read the Docs colour, rather than the pink used elsewhere in the docs, when viewed from a narrow screen / mobile.

Second, there's a small typo in the README ("let's" instead of "lets").