Fix typo in writable string test

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@44398 91177308-0d34-0410-b5e6-96231b3b80d8

test/CodeGen/writable-strings.c

@@ -1,4 +1,4 @@
-// RUN: clang -emit-llvm -fwritable-string %s
+// RUN: clang -emit-llvm -fwritable-strings %s
 
 int main() {
     char *str = "abc";