Fix README typo in the Possible theoretical foundation section

Author: willyross

README.md

@@ -128,7 +128,7 @@ Now, imagine we were to apply this procedure, and consider a statement `S` that
 
 **Case 2**: `S`'s precondition is stronger than its postcondition (e.g., `S` is an assertion). Then `S` imposes constraints on the environments in which it executes. Put another way, `S` _tests_ something. Thus, removing `S` would likely detract from the test's overarching purpose.
 
-Conversely, consider a statement `S` that satisfies (`*`). Here is why it might make sense to remove `S`. Think of `S` as _shifting_ the set of valid environments, rather than constraining them. More precisely, if `S`'s weakest precondition `P` does not imply `Q`, and if `Q` is satisfiable, the there is an assignment to `P` and `Q`'s free variables that satisfies both `P` and `Q`. If such an assignment results from each environment in which `S` is actually executed, then the necessity of `S` is called into question.
+Conversely, consider a statement `S` that satisfies (`*`). Here is why it might make sense to remove `S`. Think of `S` as _shifting_ the set of valid environments, rather than constraining them. More precisely, if `S`'s weakest precondition `P` does not imply `Q`, and if `Q` is satisfiable, then there is an assignment to `P` and `Q`'s free variables that satisfies both `P` and `Q`. If such an assignment results from each environment in which `S` is actually executed, then the necessity of `S` is called into question.
 
 The main utility of (`*`) is in helping to select the functions, macros, and method calls that Necessist ignores. Necessist ignores certain of these by default. Suppose that, for one of the frameworks, we are considering whether Necessist should ignore some function `foo`. If we imagine a predicate transformer semantics for the framework's testing language, we can ask: if statement `S` were a call to `foo`, would `S` satisfy (`*`)? If the answer is "no," then Necessist should likely ignore `foo`.