Remove type annotations from borrowck example (#2791)

These slides are on day 3, I don't think we need the explicit type
annotations at this point.

src/borrowing/borrowck.md

@@ -29,11 +29,11 @@ rule. For a given value, at any time:
 
 ```rust,editable,compile_fail
 fn main() {
-    let mut a: i32 = 10;
-    let b: &i32 = &a;
+    let mut a = 10;
+    let b = &a;
 
     {
-        let c: &mut i32 = &mut a;
+        let c = &mut a;
         *c = 20;
     }
 

src/lifetimes/lifetime-annotations.md

@@ -33,8 +33,8 @@ fn left_most(p1: &Point, p2: &Point) -> &Point {
 }
 
 fn main() {
-    let p1: Point = Point(10, 10);
-    let p2: Point = Point(20, 20);
+    let p1 = Point(10, 10);
+    let p2 = Point(20, 20);
     let p3 = left_most(&p1, &p2); // What is the lifetime of p3?
     dbg!(p3);
 }