Fix typos.

Reviewers: mclow.lists, EricWF

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D43224

llvm-svn: 324989
diff --git a/libcxx/docs/DesignDocs/CapturingConfigInfo.rst b/libcxx/docs/DesignDocs/CapturingConfigInfo.rst
index 73378a2..8810225 100644
--- a/libcxx/docs/DesignDocs/CapturingConfigInfo.rst
+++ b/libcxx/docs/DesignDocs/CapturingConfigInfo.rst
@@ -46,7 +46,7 @@
 
 Otherwise we create a custom installation rule that modifies the installed __config
 header. The rule first generates a dummy "__config_site" header containing the required
-#defines. The contents of the dummy header are then prependend to the installed
+#defines. The contents of the dummy header are then prepended to the installed
 __config header. By manually prepending the files we avoid the cost of an
 extra #include and we allow the __config header to be ignorant of the extra
 configuration all together. An example "__config" header generated when