[doc] Typo fix in NewPassManager.rst

Simple typo fix.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D112780

llvm/docs/NewPassManager.rst

@@ -63,7 +63,7 @@ can only contain function passes:
   // InstSimplifyPass is a function pass
   FPM.addPass(InstSimplifyPass());
 
-If you want add a loop pass that runs on all loops in a function to a
+If you want to add a loop pass that runs on all loops in a function to a
 ``FunctionPassManager``, the loop pass must be wrapped in a function pass
 adaptor that goes through all the loops in the function and runs the loop
 pass on each one.