[clang-tools-extra] NFC: Fix trivial typo in documents and comments

Differential Revision: https://reviews.llvm.org/D77458

clang-tools-extra/clang-tidy/fuchsia/MultipleInheritanceCheck.cpp

@@ -116,7 +116,7 @@ void MultipleInheritanceCheck::check(const MatchFinder::MatchResult &Result) {
     }
 
     if (NumConcrete > 1) {
-      diag(D->getBeginLoc(), "inheriting mulitple classes that aren't "
+      diag(D->getBeginLoc(), "inheriting multiple classes that aren't "
                              "pure virtual is discouraged");
     }
   }