You have two options here: either turn off Auto-correction entirely by going to Settings - General - Keyboard; or you can add the word cstride to the Auto-Correction dictionary.
In order to add a new word to the Auto-Correction dictionary, press Backspace as soon as the word is corrected. This results into a black bubble with your originally typed word and a few other word corrections. Select your originally typed word and it should be added to the dictionary. Alternatively, go to Settings - General - Keyboard - Text Replacement and click on + sign. Add your word, the same word, in the fields of Phrase as well as Shortcut. cstride will never be replaced by iOS again.