STYLE: fix docs

Source/cmAddLibraryCommand.h

@@ -52,7 +52,7 @@ public:
    */
   virtual const char* GetTerseDocumentation() 
     {
-    return "Add an library to the project using the specified source files.";
+    return "Add a library to the project using the specified source files.";
     }
   
   /**

Source/cmAddSubDirectoryCommand.h

@@ -71,7 +71,7 @@ public:
       "directory (the typical usage), but it may also be an absolute path. "
       "The binary_dir specifies the directory in which to place the output "
       "files. If it is a relative path it will be evaluated with respect "
-      "to the current otuput directory, but it may also be an absolute "
+      "to the current output directory, but it may also be an absolute "
       "path. If binary_dir is not specified, the value of source_dir, "
       "before expanding any relative path, will be used (the typical usage). "
       "The CMakeLists.txt file in the specified source directory will "