More tiny fixes (#1577)

src/borrowing/exercise.md

@@ -7,7 +7,7 @@ minutes: 30
 {{#include ../../third_party/rust-on-exercism/health-statistics.md}}
 
 Copy the code below to <https://play.rust-lang.org/> and fill in the missing
-methods:
+method:
 
 ```rust
 // TODO: remove this when you're done with your implementation.

src/slices-and-lifetimes/exercise.md

@@ -47,7 +47,7 @@ What remains for you is to implement the `parse_field` function.
 
 
 {{#include exercise.rs:parse_field }}
-    // 1. Read and unpack the tag.
+    // 1. Read and unpack the tag, which is a varint.
     // 2. Based on the wire type, build a Field, consuming as many bytes as
     //    necessary.
     // 3. Return the field, and any un-consumed bytes.

src/smart-pointers/exercise.md

@@ -14,7 +14,7 @@ Implement the following types, so that the given tests pass.
 ```rust,editable
 {{#include exercise.rs:types}}
 
-// Implement `new`, `insert`, and `has`.
+// Implement `new`, `insert`, `len`, and `has`.
 
 {{#include exercise.rs:tests}}
 ```