Fix some typos in the ttx usage instructions

Patch-by: Paul Flo Williams <paul@frixxon.co.uk>
Fixes: https://bugzilla.redhat.com/694387
Fixes: http://sf.net/support/tracker.php?aid=3279073

git-svn-id: svn://svn.code.sf.net/p/fonttools/code/trunk@590 4cde692c-a291-49d1-8350-778aa11640f8
diff --git a/Lib/fontTools/ttx.py b/Lib/fontTools/ttx.py
index 83bffc9..b6cc184 100644
--- a/Lib/fontTools/ttx.py
+++ b/Lib/fontTools/ttx.py
@@ -9,7 +9,7 @@
        or OpenType font file.
 
     Output files are created so they are unique: an existing file is
-       never overwrritten.
+       never overwritten.
 
     General options:
     -h Help: print this message
@@ -46,7 +46,7 @@
     -m Merge with TrueType-input-file: specify a TrueType or OpenType
        font file to be merged with the TTX file. This option is only
        valid when at most one TTX file is specified.
-    -b Don't recalc glyph boundig boxes: use the values in the TTX
+    -b Don't recalc glyph bounding boxes: use the values in the TTX
        file as-is.
 """