Fix some grammatical errors

interface/wx/dcbuffer.h

@@ -36,7 +36,7 @@
     Otherwise, wxBufferedDC can be used in the same way as any other device
     context.
 
-    There is another possible use for wxBufferedDC is to use it to maintain a
+    Another possible use for wxBufferedDC is to use it to maintain a
     backing store for the window contents. In this case, the associated @e DC
     may be @NULL but a valid backing store bitmap should be specified.
 

interface/wx/image.h

@@ -953,9 +953,8 @@ public:
         than @a threshold are replaced with the mask colour and the alpha
         channel is removed. Otherwise nothing is done.
 
-        The mask colour is chosen automatically using
-        FindFirstUnusedColour() by this function, see the overload below if you
-        this is not appropriate.
+        The mask colour is chosen automatically using FindFirstUnusedColour(),
+        see the overload below if this is not appropriate.
 
         @return Returns @true on success, @false on error.
     */