Fix doc references to 'for each' syntax to match the compiler.

The rustboot compiler expects 'for each (type v in ...)' like 'for',
rather than 'for each (type v = ...)' as given in the documentation.

Author: rillian

doc/rust.texi

@@ -1845,7 +1845,7 @@ iter range(int lo, int hi) -> int @{
 @}
 
 let int sum = 0;
-for each (int x = range(0,100)) @{
+for each (int x in range(0,100)) @{
     sum += x;
 @}
 @end example
@@ -2325,7 +2325,7 @@ iter range(int x, int y) -> int @{
   @}
 @}
 
-for each (int i = range(5,7)) @{
+for each (int i in range(5,7)) @{
   @dots{};
 @}
 @end example
@@ -3149,7 +3149,7 @@ fn read_file_lines(&str path) -> vec[str] @{
     note path;
     vec[str] r;
     file f = open_read(path);
-    for each (str &s = lines(f)) @{
+    for each (str &s in lines(f)) @{
         vec.append(r,s);
     @}
     ret r;
@@ -3282,7 +3282,7 @@ Example of a foreach loop:
 @example
 let str txt;
 let vec[str] lines;
-for each (&str s = _str.split(txt, "\n")) @{
+for each (&str s in _str.split(txt, "\n")) @{
     vec.push(lines, s);
 @}
 @end example