fix typo/grammatical error in rust.html.markdown (#4985)

rust.html.markdown

@@ -99,7 +99,7 @@ fn main() {
     // A string slice – an immutable view into another string
     // This is basically an immutable pair of pointers to a string – it doesn’t
     // actually contain the contents of a string, just a pointer to
-    // the begin and a pointer to the end of a string buffer,
+    // the beginning and a pointer to the end of a string buffer,
     // statically allocated or contained in another object (in this case, `s`).
     // The string slice is like a view `&[u8]` into `Vec<T>`.
     let s_slice: &str = &s;