[DOC] Fix typo in String#partition

Author: peterzhu2118

doc/string/partition.rdoc

@@ -29,7 +29,7 @@ If +pattern+ is a Regexp, performs the equivalent of <tt>self.match(pattern)</tt
   'hello'.partition(/x/)  # => ["hello", "", ""]
 
 If +pattern+ is not a Regexp, converts it to a string (if it is not already one),
-then performs the equivalet of <tt>self.index(pattern)</tt>
+then performs the equivalent of <tt>self.index(pattern)</tt>
 (and does _not_ set {pattern-matching global variables}[rdoc-ref:globals.md@Pattern+Matching]):
 
   'hello'.partition('h')     # => ["", "h", "ello"]