Help: Fix preset example cmakeMinimumRequired

Since commit 8bc5c896 (CMakePresets.json: Add the ability to
conditionally disable presets, 2021-03-10, v3.21.0-rc1~464^2)
the example requires presets version 3 support, which is not
available until CMake 3.21.  CMake 3.20.0 can't open v3 presets.
Make cmakeMinimumRequired compatible with the example's version.

Help/manual/presets/example.json

@@ -2,7 +2,7 @@
   "version": 3,
   "cmakeMinimumRequired": {
     "major": 3,
-    "minor": 20,
+    "minor": 21,
     "patch": 0
   },
   "configurePresets": [