Make grouping_expressions plural.

This is in line with other repeated fields and was suggested by @tokoko.

Signed-off-by: Ingo Müller <[email protected]>

proto/substrait/algebra.proto

@@ -267,7 +267,7 @@ message AggregateRel {
 
     // A list of zero or more references to grouping expressions, i.e., indices
     // into the `grouping_expression` list.
-    repeated uint32 expression_reference = 2;
+    repeated uint32 expression_references = 2;
   }
 
   message Measure {