FindProtobuf: fix documentation typo

Fix typo in documentation added by commit v3.10.0-rc1~18^2
(FindProtobuf: add flag to allow descriptor files to be generated,
2017-09-21).

Modules/FindProtobuf.cmake

@@ -101,7 +101,7 @@
 #   ``HDRS``
 #     Variable to define with autogenerated header files
 #   ``DESCRIPTORS``
-#     Variable to define with auotgenerated descriptor files, if requested.
+#     Variable to define with autogenerated descriptor files, if requested.
 #   ``EXPORT_MACRO``
 #     is a macro which should expand to ``__declspec(dllexport)`` or
 #     ``__declspec(dllimport)`` depending on what is being compiled.