Fix example in the Gnus manual

* doc/misc/gnus.texi (Score Variables): In the example showing how to
use a list of functions for gnus-score-find-score-files-find-function,
return a list of strings from the lambda rather than trying to call
the string as a function (bug#45673).

doc/misc/gnus.texi

@@ -20195,7 +20195,7 @@ Phu.
 For example, to do hierarchical scoring but use a non-server-specific
 overall score file, you could use the value
 @example
-(list (lambda (group) ("all.SCORE"))
+(list (lambda (group) (list "all.SCORE"))
       'gnus-score-find-hierarchical)
 @end example