Is it possible to display this restriction in an ER diagram?
The short answer is "no."

Entity relationship diagrams can be used to show constraints based on the relationships between entities.  However, the particular constraint you want to show (that bookings do not overlap in time) is a constraint between rows in the same table, not between two or more tables.

The only single table constraint that can be illustrated in most common entity relationship diagrams is uniqueness, since ER diagrams can illustrate a primary key.