Improvde days Operator docs for parameter (#3041)

in response to @rubaboo's [comment](https://github.com/Jermolene/TiddlyWiki5/pull/2972#issuecomment-331284559)

editions/tw5.com/tiddlers/filters/days.tid

@@ -5,7 +5,7 @@ caption: days
 op-input: a [[selection of titles|Title Selection]]
 op-neg-output: those input tiddlers in which field <<.place F>> is at least <<.place D>> days in the { future | past }
 op-output: those input tiddlers in which field <<.place F>> is <<.place D>> days in the { future | past } or any time { before | after } that, including { past | future }
-op-parameter: a number of days, defaulting to 0 { positive | negative }
+op-parameter: a { positive | negative } number of days, defaulting to 0
 op-parameter-name: D
 op-purpose: filter the input by date
 op-suffix: the name of a [[date field|Date Fields]], defaulting to <<.field modified>>