[mlir][docs] Fix pass manager document

[vmrajas]

Summary

The code example for pass manager incorrectly uses nestedFunctionPM
instead of nestedAnyPm for adding CSE and Canonicalize Passes. This diff fixes
it by changing it to nestedAnyPm.

[rriddle]

Nice, thanks!

mlir/docs/PassManagement.md

 // Nest a pass manager that operates on functions within the nested SPIRV
 // module.
 OpPassManager &nestedFunctionPM = nestedModulePM.nest<func::FuncOp>();
 nestedFunctionPM.addPass(std::make_unique<MyFunctionPass>());
 
 // Nest an op-agnostic pass manager. This will operate on any viable
 // operation, e.g. func.func, spv.func, spv.module, builtin.module, etc.
 OpPassManager &nestedAnyPM = nestedModulePM.nestAny();
-nestedFunctionPM.addPass(createCanonicalizePass());
-nestedFunctionPM.addPass(createCSEPass());
+nestedAnyPM.addPass(createCanonicalizePass());
+nestedAnyPM.addPass(createCSEPass());
 
 // Run the pass manager on the top-level module.
 ModuleOp m = ...;
 if (failed(pm.run(m)))
     ... // One of the passes signaled a failure.
 ```
 
 The above pass manager contains the following pipeline structure: