Update src/content/docs/cpp/language/preprocessor.mdx

remove "non-standard" from "non-standard extension"

Co-authored-by: A. Jiang <de34@live.cn>

Author: momo773510

src/content/docs/cpp/language/preprocessor.mdx

@@ -17,7 +17,7 @@ The preprocessing directives control the behavior of the preprocessor. Each dire
 *   the `#` character.
 *   a sequence of:
     *   a standard-defined directive name (listed [below](#Capabilities)) followed by the corresponding arguments, or
-     *   one or more <DocLink src="/cpp/language/translation_phases" anchor="Phase_3">preprocessing tokens</DocLink> where the beginning token is not a standard-defined directive name, in this case the directive is conditionally-supported with implementation-defined semantics <Revision until="C++23">(e.g. a common non-standard extension is the directive `#warning` which emits a user-defined message during compilation)</Revision>, or
+     *   one or more <DocLink src="/cpp/language/translation_phases" anchor="Phase_3">preprocessing tokens</DocLink> where the beginning token is not a standard-defined directive name, in this case the directive is conditionally-supported with implementation-defined semantics <Revision until="C++23">(e.g. a common extension is the directive `#warning` which emits a user-defined message during compilation)</Revision>, or
     *   nothing, in this case the directive has no effect.
 *   a line break.