[NFC][clang-tidy] reword diagnostic note in definitions-in-headers

`make as inline` made little sense here, so I changed the `make` to `mark`
and added `the definition` as well.

Author: 5chmidti

clang-tools-extra/clang-tidy/misc/DefinitionsInHeadersCheck.cpp

@@ -102,7 +102,7 @@ void DefinitionsInHeadersCheck::check(const MatchFinder::MatchResult &Result) {
     // inline is not allowed for main function.
     if (FD->isMain())
       return;
-    diag(FD->getLocation(), /*Description=*/"make as 'inline'",
+    diag(FD->getLocation(), "mark the definition as 'inline'",
          DiagnosticIDs::Note)
         << FixItHint::CreateInsertion(FD->getInnerLocStart(), "inline ");
   } else if (const auto *VD = dyn_cast<VarDecl>(ND)) {