Fix typo in lesson 1

fixes #4

Author: Noratrieb

lesson-1.html

@@ -31,7 +31,7 @@ <h2>Lesson 1: Basics</h2>
             Every C++ mangled symbol is prefixed with the string
             <code>_Z</code>. This signifies that this is a mangled C++ symbol.
             <code>_Z</code> starts with an underscore followed by an uppercase
-            letter. All symbols of that structures are reserved by the C
+            letter. All symbols of that structure are reserved by the C
             standard and cannot be used by programs. This ensures that there are
             no name collisions with normal C functions and mangled C++
             functions.