docs: reference: remove trailing space to fix yaml formatting

This was introduced in 41a5e0e4df, and
having the trailing whitespace causes the yamldocs generator to
switch to "compact" formatting, which makes that yaml hard to read.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit 4b35192d7c)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/commandline/cp.md

@@ -94,7 +94,7 @@ you must be explicit with a relative or absolute path, for example:
 Copy a local file into container
 
 ```console
-$ docker cp ./some_file CONTAINER:/work 
+$ docker cp ./some_file CONTAINER:/work
 ```
 
 Copy files from container to local path