[string::assign] Add missing comma in expression. Thanks to

tomalakgeretkal for reporting this.

Fixes #434

source/strings.tex

@@ -2127,7 +2127,7 @@
 Determines the effective length \tcode{rlen}
 of the string to assign as the smaller of \tcode{n} and
 \tcode{\tcode{str}.size() - \tcode{pos}} and calls
-\tcode{assign(str.data() + pos rlen)}.
+\tcode{assign(str.data() + pos, rlen)}.
 
 \pnum
 \returns