Update 0164.md

Update the format and break it to two lines for `If the expunge process takes significant time, the method **may** be a `google.longrunning.Operation` (AIP-151) instead:`

Author: wenmh-coder

aip/general/0164.md

@@ -122,7 +122,8 @@ rpc ExpungeBook(ExpungeBookRequest) returns (google.protobuf.Empty) {
 
 ### Long-running expunge
 
-If the expunge process takes significant time, the method **may** be a `google.longrunning.Operation` (AIP-151) instead:
+If the expunge process takes significant time, the method **may** be a
+`google.longrunning.Operation` (AIP-151) instead:
 
 ```proto
 // Permanently deletes a soft-deleted Book.