Remove README link to the now-defunct diff/patch packages

Author: apparentlymart

README.md

@@ -40,7 +40,6 @@ For more details, see the following documentation:
 * [Conversion to and from native Go values](./docs/gocty.md)
 * [JSON serialization](./docs/json.md)
 * [`cty` Functions system](./docs/functions.md)
-* [diff and patch for `cty` values](./docs/diff.md)
 
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