Typo s/custop/custom/

Author: stevegury

Part 3 - Taming the sequence/7. Custom operators.md

@@ -2,7 +2,7 @@
 
 RxJava offers a very large [operator set](http://reactivex.io/RxJava/javadoc/rx/Observable.html). Counting all the overloads, the number of operators on Rx is over 300. A smaller number of those is essential, meaning that you cannot achieve an Rx implementation without them. Many are there just for convenience and a self-descriptive name. For example, if `source.First(user -> user.isOnline())` didn't exist, we would still be able to do `source.filter(user -> user.isOnline()).First()`.
 
-Despite many convenience operators, the operator set of RxJava is still very basic. Rx offers the building blocks that you can combine into anything, but eventually you will want to define reuseable code for repeated cases. In standard Java, this would be done with custom classes and methods. In Rx, you would like the ability to design custop operators. For example, calculating a running average from a sequence of numbers may be very common in your financial application. That doesn't already exist in `Observable`, but you can make it yourself:
+Despite many convenience operators, the operator set of RxJava is still very basic. Rx offers the building blocks that you can combine into anything, but eventually you will want to define reuseable code for repeated cases. In standard Java, this would be done with custom classes and methods. In Rx, you would like the ability to design custom operators. For example, calculating a running average from a sequence of numbers may be very common in your financial application. That doesn't already exist in `Observable`, but you can make it yourself:
 
 ```java
 class AverageAcc {