applied typos and spelling error fixes patch from Olly Betts

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@15779 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/latex/wx/memorydc.tex

@@ -46,7 +46,7 @@ can be reselected into another memory DC.
 Constructs a new memory device context.
 
 Use the {\it Ok} member to test whether the constructor was successful
-in creating a useable device context. Don't forget to select a bitmap
+in creating a usable device context. Don't forget to select a bitmap
 into the DC before drawing on it.
 
 \membersection{wxMemoryDC::SelectObject}