Fix typo in attr.td: Avaiable -> Available (llvm#170116)

Follow up to llvm#163618

Author: badumbatish

clang/include/clang/Basic/Attr.td

@@ -508,7 +508,7 @@ def TargetMicrosoftRecordLayout : TargetArch<["x86", "x86_64", "arm", "thumb",
   let CustomCode = [{ Target.hasMicrosoftRecordLayout() }];
 }
 
-def TargetMustTailAvaiable:  TargetSpec {
+def TargetMustTailAvailable:  TargetSpec {
   let CustomCode = [{ Target.hasMustTail() }];
 }
 
@@ -1917,7 +1917,7 @@ def NoMerge : DeclOrStmtAttr {
                              "functions, statements and variables">;
 }
 
-def MustTail : StmtAttr, TargetSpecificAttr<TargetMustTailAvaiable> {
+def MustTail : StmtAttr, TargetSpecificAttr<TargetMustTailAvailable> {
   let Spellings = [Clang<"musttail">];
   let Documentation = [MustTailDocs];
   let Subjects = SubjectList<[ReturnStmt], ErrorDiag, "return statements">;