Editorial: Update a variable with set, not let (#415)

Per https://infra.spec.whatwg.org/#variables

Author: inexorabletash

index.bs

@@ -1561,9 +1561,9 @@ be updated.
 
   1. Let |value| be the [=key/value=] of |key|.
 
-  1. Let |value| be the minimum of |value| and 2<sup>53</sup> (9007199254740992).
+  1. Set |value| to the minimum of |value| and 2<sup>53</sup> (9007199254740992).
 
-  1. Let |value| be the largest integer not greater than |value|.
+  1. Set |value| to the largest integer not greater than |value|.
 
   1. Let |generator| be |store|'s [=key generator=].