Merge pull request #1240 from pierwill/patch-1

Update subtyping.md

Author: Havvy

src/subtyping.md

@@ -1,7 +1,7 @@
 # Subtyping and Variance
 
 Subtyping is implicit and can occur at any stage in type checking or
-inference. Subtyping in Rust is very restricted and occurs only due to
+inference. Subtyping is restricted to two cases:
 variance with respect to lifetimes and between types with higher ranked
 lifetimes. If we were to erase lifetimes from types, then the only subtyping
 would be due to type equality.