cmake.py: Improve documentation (#25467)

Add missing `self.` prefixes when calling `define_from_variant`

lib/spack/spack/build_systems/cmake.py

@@ -254,9 +254,9 @@ def define_from_variant(self, cmake_var, variant=None):
 
             .. code-block:: python
 
-                [define_from_variant('BUILD_SHARED_LIBS', 'shared'),
-                 define_from_variant('CMAKE_CXX_STANDARD', 'cxxstd'),
-                 define_from_variant('SWR')]
+                [self.define_from_variant('BUILD_SHARED_LIBS', 'shared'),
+                 self.define_from_variant('CMAKE_CXX_STANDARD', 'cxxstd'),
+                 self.define_from_variant('SWR')]
 
             will generate the following configuration options: