Windows in an screen session? In a screen session? Doesn't matter?

Hello,
Referring to 
https://www.gnu.org/software/screen/manual/html_node/Selecting.html#Selecting 
, openning sentence is 

    This section describes the commands for switching between windows in 
    an screen session.

Does it matter if it is `an screen' or `a screen'?

--
u34