Help: Fix typo in cmake_path(CMAKE_PATH) command signature

Fix a typo accidentally added by commit eb583b0a (cmake_path command:
path management, 2020-07-23).  Remove a stray `]` character that breaks
the pygments lexer.

Help/command/cmake_path.rst

@@ -443,7 +443,7 @@ Conversion
 
 .. code-block:: cmake
 
-    cmake_path(CMAKE_PATH <path> [NORMALIZE] <input>])
+    cmake_path(CMAKE_PATH <path> [NORMALIZE] <input>)
 
 Converts a native ``<input>`` path into cmake-style path with forward-slashes
 (``/``). On Windows, the long filename marker is taken into account.