Docs: Correct some spelling error

Change-Id: I0a7006dc816b204284bce2793187942250720a18
Signed-off-by: Edison Ai <edison.ai@arm.com>
diff --git a/docs/design_documents/source_structure.rst b/docs/design_documents/source_structure.rst
index 1b71215..42641a1 100644
--- a/docs/design_documents/source_structure.rst
+++ b/docs/design_documents/source_structure.rst
@@ -6,8 +6,8 @@
 :Contact: tf-m@lists.trustedfirmware.org
 
 .. note::
-  Refernce the document :doc:`Glossary </docs/reference/glossary>` for terms and
-  abbreviations.
+  Reference the document :doc:`Glossary </docs/reference/glossary>` for terms
+  and abbreviations.
 
 ************
 Introduction
@@ -29,7 +29,7 @@
 The description of the source structure is broken down into subsections, each
 subsection introduces one detailed folder.
 
-Root Direcotry
+Root Directory
 ==============
 This table describes the structure under the root directory with part of
 possible folders. Note that the ``Detailed`` field indicates if the folder is