For 802.11n, you have a choice of 20MHz or 40MHz Channel Width.
With the 1x1 (1 transmit antenna, 1 receiver antenna) WiFi setup for all variants of the ProBook 450 G2, this means that you get 72 Mbps maximum in 20MHz mode, or 150 Mbps maximum in 40MHz mode. (With other devices with more antennas, & with a compatible router, you'd get double that speed or more as the max possible).
Assuming the 802.11b/g/n card used for your ProBook configuration can connect with a 40MHz-wide N channel, you have to set the Time Capsule up with a 40 MHz-wide channel.
But, as Apple describes in its documents (and as is the case for all WiFi devices), a wider channel allows for more interference, so you may not see any gains in terms of effective connection speed if you switch to the 40 MHz setting.