Replace tabs with spaces for proper formatting

[pombredanne]

Without this small change, the license looks weird if tabs do not align exactly with 4 spaces.

[nobu]

8 spaces?

[pombredanne]

@nobu correct. There are just a few odd tabs lost in that file and rest if using spaces for formatting. Which makes the text look weird if your tab does not expand to 8 spaces not 4. This makes the formatting consistent with the rest of the file using spaces only.