[mlir][docs] Add missing parentheses in example code on walkers.

[ingomueller-net]

Summary

getFunction was missing parentheses.

[mehdi_amini]

Thanks for fixing the doc :)

(do you have commit access?)

[ingomueller-net]

Yes :) Landed!

mlir/docs/Tutorials/UnderstandingTheIRStructure.md

```c++
   });
 ```
 
 The provided callback can be specialized to filter on a particular type of
 Operation, for example the following will apply the callback only on `LinalgOp`
 operations nested inside the function:
 
 ```c++
-  getFunction.walk([](LinalgOp linalgOp) {
+  getFunction().walk([](LinalgOp linalgOp) {
     // process LinalgOp `linalgOp`.
   });
 ```
 
 Finally, the callback can optionally stop the walk by returning a
 `WalkResult::interrupt()` value. For example the following walk will find all
 `AllocOp` nested inside the function and interrupt the traversal if one of them
 does not satisfy a criteria: