[mlir][gpu][docs] fix incorrect syntax for gpu.launch (#76381)

Per the code:

https://github.com/llvm/llvm-project/blob/5c39b8d1a86cc0c92acd438d4799d19e67ae70db/mlir/include/mlir/Dialect/GPU/IR/GPUOps.td#L805

And the usage:

https://github.com/llvm/llvm-project/blob/5c39b8d1a86cc0c92acd438d4799d19e67ae70db/mlir/lib/Dialect/GPU/IR/GPUDialect.cpp#L869

The keyword should be `blocks` rather than `block`. The documentation of
the syntax is out of date.