Corrected incorrect description (#226)

The description of "count" from basic_string::assign() was probably copypasted from basic_string::append(), but it's not clearly true (verified by test and documentation elsewhere - http://www.cplusplus.com/reference/string/string/assign/).

Author: Vizor

docs/standard-library/basic-string-class.md

@@ -361,7 +361,7 @@ basic_string<CharType, Traits, Allocator>& assign(
  A pointer to the characters of the C-string to be assigned to the target string.
 
 `count`
- The number of characters to be appended, at most, from the source string.
+ The number of characters to be assigned, from the source string.
 
 `str`
  The source string whose characters are to be assigned to the target string.