Update String.h

Summary:
Fixed typo in doc comment related to `void toLowerAscii(char* str, size_t length)`.
This fixes the annoying xCode's warning `Parameter 'len' not found in the function declaration`.
Closes https://github.com/facebook/folly/pull/670

Reviewed By: yfeldblum

Differential Revision: D5815034

Pulled By: Orvid

fbshipit-source-id: 9580cd0bbd4924d4ed461a5435f525316e5c285a

folly/String.h

@@ -618,7 +618,7 @@ std::string stripLeftMargin(std::string s);
  * Leaves all other characters unchanged, including those with the 0x80
  * bit set.
  * @param str String to convert
- * @param len Length of str, in bytes
+ * @param length Length of str, in bytes
  */
 void toLowerAscii(char* str, size_t length);