Fix typo in doc

Author: kumy

README.md

@@ -94,7 +94,7 @@ $pagination->pagePrefix(text);
 $pagination->pageSuffix(text);
 
 # sets the flag to retain the query string for each page link.
-$pagination->retrainQueryString();
+$pagination->retainQueryString();
 
 # sets the flag to ignore the query string when building the links
 $pagination->dismissQueryString();
@@ -124,4 +124,4 @@ Licensed under the MIT license
 
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