Fix Incorrect Quote In Docker Executor Example

What does this MR do?

The corrected line opened with a regular quote (') but closed with a back-tick (`)
I believe this is invalid bash syntax

Are there points in the code the reviewer needs to double check?

Probably not

Why was this MR needed?

Fixes error when example is copied and pasted

What are the relevant issue numbers?

None

Screenshots (if relevant)

None