Update existential-types.md

Author: oli-obk

src/existential-types.md

@@ -7,7 +7,7 @@ type.
 
 In the language they are expressed via
 
-```rust
+```
 existential type Foo: Bar;
 ```
 
@@ -17,7 +17,7 @@ the `Bar` trait's interface.
 Since there needs to be a concrete background type, you can currently
 express that type by using the existential type in a "defining use site".
 
-```rust
+```
 struct Struct;
 impl Bar for Struct { /* stuff */ }
 fn foo() -> Foo {
@@ -45,4 +45,4 @@ of existential types to be defining use sites.
 ### Associated existential types
 
 Associated existential types can be defined by any other associated item
-on the same trait `impl` or a child of these associated items.
+on the same trait `impl` or a child of these associated items.