docs: fix grammar

Author: kgryte

lib/node_modules/@stdlib/slice/base/seq2slice/README.md

@@ -114,7 +114,7 @@ v = s.step;
 // returns -1
 ```
 
-The function returns an error object if provided in invalid subsequence string.
+The function returns an error object if provided an invalid subsequence string.
 
 ```javascript
 var s = seq2slice( '1:2:3:4', 10, false );