docs/reference: explain "colon"

Colon may not be something non-native English readers
know about, so explain the symbol in the running text.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/commandline/node_ps.md

@@ -130,7 +130,7 @@ output the data exactly as the template declares or, when using the
 `table` directive, includes column headers as well.
 
 The following example uses a template without headers and outputs the
-`Name` and `Image` entries separated by a colon for all tasks:
+`Name` and `Image` entries separated by a colon (`:`) for all tasks:
 
 ```bash
 $ docker node ps --format "{{.Name}}: {{.Image}}"