[clang-tidy][docs][NFC] Update URL and docs of PostfixOperatorCheck

The docs URL was dangling, and the docs suggested that it has no fixits,
but it actually had.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D121372

clang-tools-extra/clang-tidy/cert/PostfixOperatorCheck.h

@@ -19,7 +19,7 @@ namespace cert {
 /// object.
 ///
 /// For the user-facing documentation see:
-/// http://clang.llvm.org/extra/clang-tidy/checks/cert-postfix-operator.html
+/// https://clang.llvm.org/extra/clang-tidy/checks/cert-dcl21-cpp.html
 class PostfixOperatorCheck : public ClangTidyCheck {
 public:
   PostfixOperatorCheck(StringRef Name, ClangTidyContext *Context)

clang-tools-extra/docs/clang-tidy/checks/list.rst

@@ -113,7 +113,7 @@ Clang-Tidy Checks
    `bugprone-unused-return-value <bugprone-unused-return-value.html>`_,
    `bugprone-use-after-move <bugprone-use-after-move.html>`_,
    `bugprone-virtual-near-miss <bugprone-virtual-near-miss.html>`_, "Yes"
-   `cert-dcl21-cpp <cert-dcl21-cpp.html>`_,
+   `cert-dcl21-cpp <cert-dcl21-cpp.html>`_, "Yes"
    `cert-dcl50-cpp <cert-dcl50-cpp.html>`_,
    `cert-dcl58-cpp <cert-dcl58-cpp.html>`_,
    `cert-env33-c <cert-env33-c.html>`_,