[mlir][Linalg][Doc] Minor fix of generic op matmul example

[limo1996]

Summary

Example of Matmul implementation in linalg.generic operation contained few mistakes that can puzzle new startes when trying to run the example.

[ftynse]

Please don't use too many [tags]. Many people put [mlir] in order to tell apart the commits in git log, sometimes [mlir][Linalg] can be appropriate, but here something like [mlir] Fix linalg.generic matmul example in the doc would say same thing in a more readable way.

mlir/include/mlir/Dialect/Linalg/IR/LinalgStructuredOps.td

Defining a #matmul_trait attribute in MLIR can be done as follows:
         n_views = [2, 1],
         iterator_types = ["parallel", "parallel", "reduction"]
       }
       ```
 
     And can be reused in multiple places as:
       ```mlir
       linalg.generic #matmul_trait %A, %B, %C [other-attributes] {
-        (%a: f32, %b: f32, %c: f32) :
+        ^bb0(%a: f32, %b: f32, %c: f32) :
           %d = mulf %a, %b: f32
           %e = addf %c, %d: f32
-          linalg_yield %e : f32
+          linalg.yield %e : f32
       } : memref<?x?xf32, stride_specification>,
           memref<?x?xf32, stride_specification>,
           memref<?x?xf32, stride_specification>
       ```
 
     This may lower to either:
       ```mlir
       call @linalg_matmul(%A, %B, %C) :