Fix up links

mdbook and rustdoc generate links differently, so we need to change all
these links.

src/lifetime-mismatch.md

@@ -78,4 +78,4 @@ TODO: other common problems? SEME regions stuff, mostly?
 
 
 
-[ex2]: lifetimes.html#example-aliasing-a-mutable-reference
+[ex2]: lifetimes.html#Example%3A%20aliasing%20a%20mutable%20reference

src/repr-rust.md

@@ -151,4 +151,4 @@ use fairly elaborate algorithms to cache bits throughout nested types with
 special constrained representations. As such it is *especially* desirable that
 we leave enum layout unspecified today.
 
-[dst]: exotic-sizes.html#dynamically-sized-types-dsts
+[dst]: exotic-sizes.html#Dynamically%20Sized%20Types%20(DSTs)