Help: Replace CMP0054 example to show when it makes a difference

The previous example for CMP0054 did not show how critical the policy is
since the result is the same in both cases (NEW and OLD behavior).
Instead show an example when the policy does make a difference.

Help/policy/CMP0054.rst

@@ -16,29 +16,33 @@ Given the following partial example:
 
 ::
 
-  set(MONKEY 1)
-  set(ANIMAL MONKEY)
+  set(A E)
+  set(E "")
 
-  if("${ANIMAL}" STREQUAL "MONKEY")
+  if("${A}" STREQUAL "")
+    message("Result is TRUE before CMake 3.1 or when CMP0054 is OLD")
+  else()
+    message("Result is FALSE in CMake 3.1 and above if CMP0054 is NEW")
+  endif()
 
 After explicit expansion of variables this gives:
 
 ::
 
-  if("MONKEY" STREQUAL "MONKEY")
+  if("E" STREQUAL "")
 
 With the policy set to ``OLD`` implicit expansion reduces this semantically to:
 
 ::
 
-  if("1" STREQUAL "1")
+  if("" STREQUAL "")
 
 With the policy set to ``NEW`` the quoted arguments will not be
 further dereferenced:
 
 ::
 
-  if("MONKEY" STREQUAL "MONKEY")
+  if("E" STREQUAL "")
 
 This policy was introduced in CMake version 3.1.
 CMake version |release| warns when the policy is not set and uses