runtime(openPlugin): fix unclosed parenthesis in GetWordUnderCursor()

runtime(openPlugin): fix unclosed parenthesis in GetWordUnderCursor()

Problem:

Saw the following error when invoking gx mapping with cursor on a URL:

    Error detected while compiling function <SNR>95_GetWordUnderCursor:
    line    7:
    E1123: Missing comma before argument: return expand(user_var)
    E116: Invalid arguments for function dist#vim9#Open

Solution:

The line before "return expand(...)" has two open parentheses but only
one close parenthesis. Add another close parenthesis.

closes: #16553

Signed-off-by: Morton Fox <github@qslw.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>