Fix typo

Author: huachaohuang

src/doc/trpl/documentation.md

@@ -517,7 +517,7 @@ can be useful when changing some options, or when writing a macro.
 
 ### Re-exports
 
-`rustdoc` will show the documentation for a publc re-export in both places:
+`rustdoc` will show the documentation for a public re-export in both places:
 
 ```ignore
 extern crate foo;