Work around doc_link_with_quotes pedantic clippy lint

    warning: possible intra-doc link using quotes instead of backticks
      --> serde/src/de/mod.rs:67:41
       |
    67 | //!    - Rc\<T\>&emsp;*(if* features = ["rc"] *is enabled)*
       |                                         ^^^^
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#doc_link_with_quotes
       = note: `-W clippy::doc-link-with-quotes` implied by `-W clippy::pedantic`
       = help: to override `-W clippy::pedantic` add `#[allow(clippy::doc_link_with_quotes)]`

    warning: possible intra-doc link using quotes instead of backticks
      --> serde/src/de/mod.rs:68:42
       |
    68 | //!    - Arc\<T\>&emsp;*(if* features = ["rc"] *is enabled)*
       |                                          ^^^^
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#doc_link_with_quotes

    warning: possible intra-doc link using quotes instead of backticks
      --> serde/src/ser/mod.rs:64:41
       |
    64 | //!    - Rc\<T\>&emsp;*(if* features = ["rc"] *is enabled)*
       |                                         ^^^^
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#doc_link_with_quotes

    warning: possible intra-doc link using quotes instead of backticks
      --> serde/src/ser/mod.rs:65:42
       |
    65 | //!    - Arc\<T\>&emsp;*(if* features = ["rc"] *is enabled)*
       |                                          ^^^^
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#doc_link_with_quotes

serde/src/de/mod.rs

@@ -64,8 +64,8 @@
 //!    - RefCell\<T\>
 //!    - Mutex\<T\>
 //!    - RwLock\<T\>
-//!    - Rc\<T\>&emsp;*(if* features = ["rc"] *is enabled)*
-//!    - Arc\<T\>&emsp;*(if* features = ["rc"] *is enabled)*
+//!    - Rc\<T\>&emsp;*(if* features = \["rc"\] *is enabled)*
+//!    - Arc\<T\>&emsp;*(if* features = \["rc"\] *is enabled)*
 //!  - **Collection types**:
 //!    - BTreeMap\<K, V\>
 //!    - BTreeSet\<T\>

serde/src/ser/mod.rs

@@ -61,8 +61,8 @@
 //!    - RefCell\<T\>
 //!    - Mutex\<T\>
 //!    - RwLock\<T\>
-//!    - Rc\<T\>&emsp;*(if* features = ["rc"] *is enabled)*
-//!    - Arc\<T\>&emsp;*(if* features = ["rc"] *is enabled)*
+//!    - Rc\<T\>&emsp;*(if* features = \["rc"\] *is enabled)*
+//!    - Arc\<T\>&emsp;*(if* features = \["rc"\] *is enabled)*
 //!  - **Collection types**:
 //!    - BTreeMap\<K, V\>
 //!    - BTreeSet\<T\>