Fix typo in tutorial.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@238885 91177308-0d34-0410-b5e6-96231b3b80d8

docs/LibASTMatchersTutorial.rst

@@ -169,7 +169,7 @@ You should now be able to run the syntax checker, which is located in
 
 .. code-block:: console
 
-      cat "int main() { return 0; }" > test.cpp
+      echo "int main() { return 0; }" > test.cpp
       bin/loop-convert test.cpp --
 
 Note the two dashes after we specify the source file. The additional