fix typo in flex.md (#24431)

docs/4.0/utilities/flex.md

@@ -215,7 +215,7 @@ Responsive variations also exist for `align-self`.
 
 ## Auto margins
 
-Flexbox can do some pretty awesome things when you mix flex alignments with auto margins. Shown below is are three examples of controlling flex items via auto margins: default (no auto margin), pushing two items to the right (`.mr-auto`), and pushing two items to the left (`.ml-auto`).
+Flexbox can do some pretty awesome things when you mix flex alignments with auto margins. Shown below are three examples of controlling flex items via auto margins: default (no auto margin), pushing two items to the right (`.mr-auto`), and pushing two items to the left (`.ml-auto`).
 
 **Unfortunately, IE10 and IE11 do not properly support auto margins on flex items whose parent has a non-default `justify-content` value.** [See this StackOverflow answer](https://stackoverflow.com/a/37535548) for more details.