fix typo in IndepthTutorial.md

docs/markdown/IndepthTutorial.md

@@ -65,7 +65,7 @@ there and execute it. The last few lines are a stanza to generate a
 `pkg-config` file. We'll skip that for now and come back to it at the
 end of this document.
 
-The first subdirectory we go into is `include`. In it we have a a
+The first subdirectory we go into is `include`. In it we have a
 header file for the library that we want to install. This requires one
 line.