Update date reference about infer context variables

Signed-off-by: Yuki Okushi <jtitor@2k36.org>

src/type-inference.md

@@ -68,7 +68,7 @@ inference works, or perhaps this blog post on
 [Unification in the Chalk project]: http://smallcultfollowing.com/babysteps/blog/2017/03/25/unification-in-chalk-part-1/
 
 All told, the inference context stores five kinds of inference variables
-(as of <!-- date-check --> June 2021):
+(as of <!-- date-check --> March 2023):
 
 - Type variables, which come in three varieties:
   - General type variables (the most common). These can be unified with any