Fix comment typos in sources

No real changes.

Closes https://github.com/wxWidgets/wxWidgets/pull/2541

include/wx/strconv.h

@@ -143,7 +143,7 @@ public:
 
     // return true if the converter's charset is UTF-8, i.e. char* strings
     // decoded using this object can be directly copied to wxString's internal
-    // storage without converting to WC and than back to UTF-8 MB string
+    // storage without converting to WC and then back to UTF-8 MB string
     virtual bool IsUTF8() const { return false; }
 
     // The old conversion functions. The existing classes currently mostly