Fix minor typo in folly/Conv.h

Summary: 'flating point' => 'floating point'

Closes https://github.com/facebook/folly/issues/241

Reviewed By: yfeldblum

Differential Revision: D2685280

fb-gh-sync-id: 2e9c2b5419d0c631af4a32c5ab6b7366033ca34c

folly/Conv.h

@@ -1229,7 +1229,7 @@ to(StringPiece src) {
  ******************************************************************************/
 
 /**
- * Checked conversion from integral to flating point and back. The
+ * Checked conversion from integral to floating point and back. The
  * result must be convertible back to the source type without loss of
  * precision. This seems Draconian but sometimes is what's needed, and
  * complements existing routines nicely. For various rounding