another wording fix

proof-of-spacetime.md

@@ -11,7 +11,7 @@ This document describes Rational-PoSt, the Proof-of-Spacetime used in Filecoin.
 | Name | Value |Description |
 |------|-------|------------|
 | `POST_PROVING_PERIOD` | `2880` blocks  (~24h) | The time interval in which a PoSt has to be submitted. |
-| `POST_CHALLENGE_TIME` | `240` blocks (~2h) | The time offset at which the actual work of generating the PoSt should be started. This is some delta before the end of the `Proving Period`, and as such less then a single `Proving Period`. |
+| `POST_CHALLENGE_TIME` | `240` blocks (~2h) | The time offset at which the actual work of generating the PoSt **can not** be started earlier than. This is some delta before the end of the `Proving Period`, and as such less then a single `Proving Period`. |
 
 {{% notice todo %}}
 **TODO**: The above values are tentative and need both backing from research as well as detailed reasoning why we picked them.