Fix syntax error in lsp-treemacs-generic.el (#154)

Co-authored-by: 813 <813gan@protonmail.com>

Author: 813gan

lsp-treemacs-generic.el

@@ -86,7 +86,7 @@
        (if expanded?  "▾ " "▸ ")
      "  ")
    (or (plist-get item :icon-literal)
-       (if-let ((icon (plist-get item :icon)))
+       (if-let (icon (plist-get item :icon))
            (treemacs-get-icon-value
             icon
             nil