Fix type in benchmarks/merge.py (pydata#7235)

I don't think this affects what is displayed that is determined by param_names

asv_bench/benchmarks/merge.py

@@ -13,6 +13,6 @@ def setup(self, existing_elements):
             d[f"var{i}"] = i
         self.dataset = xr.merge([d])
 
-    def time_variable_insertion(self, existin_elements):
+    def time_variable_insertion(self, existing_elements):
         dataset = self.dataset
         dataset["new_var"] = 0