Update MAVEN_OPTS environment variables description

Under "Maven 3.x" task configuration, in order to have an external property loaded correctly, it is required to use the following syntax:

MAVEN_OPTS="-Xmx1024m" "-Dmy.property=${bamboo.myProperty}"

Please, notice the space between the two properties.

At moment, the current description is represented as per following:

Extra environment variables. e.g. MAVEN_OPTS="-Xmx256m -Xms128m". You can add multiple parameters separated by a space.