Replace deprecated missing_doc attribute.

src/doc/reference.md

@@ -2100,15 +2100,15 @@ plugins](guide-plugin.html#lint-plugins) can provide additional lint checks.
 ```{.ignore}
 mod m1 {
     // Missing documentation is ignored here
-    #[allow(missing_doc)]
+    #[allow(missing_docs)]
     pub fn undocumented_one() -> int { 1 }
 
     // Missing documentation signals a warning here
-    #[warn(missing_doc)]
+    #[warn(missing_docs)]
     pub fn undocumented_too() -> int { 2 }
 
     // Missing documentation signals an error here
-    #[deny(missing_doc)]
+    #[deny(missing_docs)]
     pub fn undocumented_end() -> int { 3 }
 }
 ```
@@ -2117,16 +2117,16 @@ This example shows how one can use `allow` and `warn` to toggle a particular
 check on and off.
 
 ```{.ignore}
-#[warn(missing_doc)]
+#[warn(missing_docs)]
 mod m2{
-    #[allow(missing_doc)]
+    #[allow(missing_docs)]
     mod nested {
         // Missing documentation is ignored here
         pub fn undocumented_one() -> int { 1 }
 
         // Missing documentation signals a warning here,
         // despite the allow above.
-        #[warn(missing_doc)]
+        #[warn(missing_docs)]
         pub fn undocumented_two() -> int { 2 }
     }
 
@@ -2139,10 +2139,10 @@ This example shows how one can use `forbid` to disallow uses of `allow` for
 that lint check.
 
 ```{.ignore}
-#[forbid(missing_doc)]
+#[forbid(missing_docs)]
 mod m3 {
     // Attempting to toggle warning signals an error here
-    #[allow(missing_doc)]
+    #[allow(missing_docs)]
     /// Returns 2.
     pub fn undocumented_too() -> int { 2 }
 }