Add missing whitespace to output of reserve-network-port

[albers]

The log message of the reserve-network-port goal has no whitespace between the literal "Port assigned" and the port number.

Example output without this change:

[INFO] --- build-helper-maven-plugin:3.2.0:reserve-network-port (reserve-network-port) @ some-module ---
[INFO] Port assigned20993

With this change:

[INFO] --- build-helper-maven-plugin:3.2.0:reserve-network-port (reserve-network-port) @ some-module ---
[INFO] Port assigned 20993

[stefanseifert]

lgtm - thanks