fix: fennel set_form grammar structure (#821)

The fennel grammar was updated to [remove
binding_pair](alexmozaidze/tree-sitter-fennel@49293d2)
from set.

Without this trying to use the queries gives an "impossible pattern"
error.

Author: micampe

queries/fennel/textobjects.scm

@@ -102,9 +102,8 @@
     rhs: (_) @assignment.rhs) @assignment.inner) @assignment.outer
 
 (set_form
-  (binding_pair
-    lhs: (_) @assignment.lhs
-    rhs: (_) @assignment.rhs) @assignment.inner) @assignment.outer
+  lhs: (_) @assignment.lhs @assignment.inner
+  rhs: (_) @assignment.rhs @assignment.inner) @assignment.outer
 
 (let_vars
   (binding_pair