fix captialization issue in CMakeLists.txt
diff --git a/docs/cmake.rst b/docs/cmake.rst
index 0298303..d405bc3 100644
--- a/docs/cmake.rst
+++ b/docs/cmake.rst
@@ -48,7 +48,7 @@
     set(Python_ADDITIONAL_VERSIONS 3.4 3.5 3.6 3.7)
     if (NOT ${EXAMPLE_PYTHON_VERSION} STREQUAL "")
       find_package(PythonLibs ${EXAMPLE_PYTHON_VERSION} EXACT)
-      if (NOT PythonLibs_FOUND)
+      if (NOT PYTHONLIBS_FOUND)
         find_package(PythonLibs ${EXAMPLE_PYTHON_VERSION} REQUIRED)
       endif()
     else()