Change to avoid repeated "is"

Author: ykomatsu

src/doc/trpl/error-handling.md

@@ -279,11 +279,11 @@ fn extension(file_name: &str) -> Option<&str> {
 }
 ```
 
-One other pattern that we find it very common is assigning a default value to
-the case when an `Option` value is `None`. For example, maybe your program
-assumes that the extension of a file is `rs` even if none is present. As you
-might imagine, the case analysis for this is not specific to file
-extensions - it can work with any `Option<T>`:
+One other pattern we commonly find is assigning a default value to the case
+when an `Option` value is `None`. For example, maybe your program assumes that
+the extension of a file is `rs` even if none is present. As you might imagine,
+the case analysis for this is not specific to file extensions - it can work
+with any `Option<T>`:
 
 ```rust
 fn unwrap_or<T>(option: Option<T>, default: T) -> T {