Update 10.collect.md (#1186)

lower case

Author: HarrisChu

docs-2.0/3.ngql-guide/6.functions-and-expressions/10.collect.md

@@ -28,7 +28,7 @@ nebula> UNWIND [1, 2, 1] AS a \
 nebula> UNWIND [1, 2, 1] AS a \
         RETURN a, collect(a), size(collect(a));
 +---+------------+------------------+
-| a | collect(a) | size(COLLECT(a)) |
+| a | collect(a) | size(collect(a)) |
 +---+------------+------------------+
 | 2 | [2]        | 1                |
 | 1 | [1, 1]     | 2                |