In my version of Bott and Tu, the example reads (emphasis mine):

Moreover, if $V \subset U$ is an inclusion of CONTRACTIBLE open sets, then $\rho^U_V: H^q(\pi^{-1} U) \to H^q( \pi^{-1} V)$ is an isomorphism.

Looks like this is the original edition, copyright 1982, 4th printing.