Replace his with their in reference to reader (#665)

The reader should not be assumed male; I'm a developer and user,
I'm not male.  Makes documentation's language gender neutral to
make it more welcoming to people that do not use he/him pronouns.

src/theme/mod.rs

@@ -36,7 +36,7 @@ pub static FONT_AWESOME_OTF: &'static [u8] = include_bytes!("_FontAwesome/fonts/
 
 
 /// The `Theme` struct should be used instead of the static variables because
-/// the `new()` method will look if the user has a theme directory in his
+/// the `new()` method will look if the user has a theme directory in their
 /// source folder and use the users theme instead of the default.
 ///
 /// You should only ever use the static variables directly if you want to

src/theme/searcher/searcher.js

@@ -4,7 +4,7 @@ window.search = window.search || {};
     // Search functionality
     //
     // You can use !hasFocus() to prevent keyhandling in your key
-    // event handlers while the user is typing his search.
+    // event handlers while the user is typing their search.
 
     if (!Mark || !elasticlunr) {
         return;