Fixed small typo in BufRead comments

`BufRead` comments, in the `Seek` trait	implementation,	was talking
about allocating 8 *ebibytes*. It was a	typo, the correct unit is
*exbibytes*, since *ebibytes* don't even exist.	The calculation	is
correct, though.

src/libstd/io/buffered.rs

@@ -231,7 +231,7 @@ impl<R: Seek> Seek for BufReader<R> {
         if let SeekFrom::Current(n) = pos {
             let remainder = (self.cap - self.pos) as i64;
             // it should be safe to assume that remainder fits within an i64 as the alternative
-            // means we managed to allocate 8 ebibytes and that's absurd.
+            // means we managed to allocate 8 exbibytes and that's absurd.
             // But it's not out of the realm of possibility for some weird underlying reader to
             // support seeking by i64::min_value() so we need to handle underflow when subtracting
             // remainder.