[csharp] Fix csharp doc comments

[tonydnewell]

Fix C# document comments when there is an extra / at the start of a comment line.
See #35905

Related to protocolbuffers/protobuf#15955 which also fixes the issue in protoc compiler.

[tonydnewell]

@apolcyn @jskeet for review

[apolcyn]

@tonydnewell clang format complains:

printer->Print("///\n");
       }
       last_was_empty = false;
-      // If the comment has an extra slash at the start then this can cause the C# compiler
-      // to complain when generating the XML documentation
-      // Issue [https://github.com/grpc/grpc/issues/35905](https://www.google.com/url?q=https://github.com/grpc/grpc/issues/35905&sa=D)
+      // If the comment has an extra slash at the start then this can cause the
+      // C# compiler to complain when generating the XML documentation Issue
+      // [https://github.com/grpc/grpc/issues/35905](https://www.google.com/url?q=https://github.com/grpc/grpc/issues/35905&sa=D)
       if (line[0] == '/') {
-        line.replace(0,1,"/");
+        line.replace(0, 1, "/");
       }
       printer->Print("///$line$\n", "line", line);
     }