variable.md: clarify 'trim' example

- Use the variable name 'var' instead of 'url' since the latter is also
  a function name and that may confuse the user.

Closes https://github.com/curl/curl/pull/16346

docs/cmdline-opts/variable.md

@@ -71,7 +71,7 @@ removes all leading and trailing white space.
 
 Example:
 
-    curl --expand-url https.//example.com/{{url:trim}}
+    curl --expand-url https://example.com/{{var:trim}}
 
 ## json
 outputs the content using JSON string quoting rules.