Make grpc.patch instructions more accurate. (tensorflow#5795)

As noted in a comment for tensorflow#5793, the instructions for generating grpc.patch would be more accurate if they referred to the commit that corresponds to the grpc archive instead of referring to the nearest tag. I've updated the instructions to refer to the commit.

I tested by following the updated instructions and copying grpc.patch to my tensorboard repo. git did not find a diff in the generated files.

Author: bmd3k

WORKSPACE

@@ -149,14 +149,15 @@ http_archive(
         # ```
         # $ git clone https://github.com/grpc/grpc.git
         # $ cd grpc
-        # $ git checkout tags/v1.27.0-pre1 -b my-patch
+        # $ git checkout b54a5b338637f92bfcf4b0bc05e0f57a5fd8fadd -b my-patch
         # $ git cherry-pick dbe73c9004e483d24168c220cd589fe1824e72bc
         # $ git diff HEAD~1 > grpc.patch
         # ```
         #
-        # Note that we choose tags/v1.27.0-pre1 as the base since the version in
-        # the archive is 1.27.0-dev. It's not necessarily an exact match but is
-        # the best guess we can make for a match.
+        # Note that we choose b54a5b338637f92bfcf4b0bc05e0f57a5fd8fadd as the
+        # base since it matches the archive (the commit number is in the archive
+        # file's name). There is no exact corresponding tag to use but the
+        # nearest might be v1.27.0-pre1.
         "//third_party:grpc.patch",
     ],
     sha256 = "b956598d8cbe168b5ee717b5dafa56563eb5201a947856a6688bbeac9cac4e1f",
@@ -166,7 +167,7 @@ http_archive(
         # Currently we can't upgrade gRPC past 1.30.0 without also bumping protobuf to 3.12.0+:
         # https://github.com/grpc/grpc/issues/23311.
         #
-        # Inspecting the contents of this archive, the version is 1.27.0-dev.
+        # Inspecting the contents of this archive, the version is v1.27.0-dev.
         "http://mirror.tensorflow.org/github.com/grpc/grpc/archive/b54a5b338637f92bfcf4b0bc05e0f57a5fd8fadd.tar.gz",
         "https://github.com/grpc/grpc/archive/b54a5b338637f92bfcf4b0bc05e0f57a5fd8fadd.tar.gz",
     ],