docs: Use a better input example in the streaming section (#3322)

The introduction says all the usage examples (e.g. of truncate_stream,
etc) in the section use [0,[1]] as input, but they actually use [1,[2]].
["a",["b"]] seems better than [1,[2]] as the string values won't be
confusable with the indexes in <path>.

tests/man.test

@@ -942,14 +942,14 @@ def while(cond; update): def _while: if cond then ., (update | _while) else empt
 1
 [1,2,4,8,16,32,64]
 
-truncate_stream([[0],1],[[1,0],2],[[1,0]],[[1]])
+truncate_stream([[0],"a"],[[1,0],"b"],[[1,0]],[[1]])
 1
-[[0],2]
+[[0],"b"]
 [[0]]
 
-fromstream(1|truncate_stream([[0],1],[[1,0],2],[[1,0]],[[1]]))
+fromstream(1|truncate_stream([[0],"a"],[[1,0],"b"],[[1,0]],[[1]]))
 null
-[2]
+["b"]
 
 . as $dot|fromstream($dot|tostream)|.==$dot
 [0,[1,{"a":1},{"b":2}]]