bip3: Change License example to CC0-1.0 OR MIT

The actual reason why I suggest this is that I think that's a great
default choice for a new BIP, so it's a perfect example. CC0-1.0 is a
great liberal choice for the BIP document (and test vectors etc.), and
MIT is the common choice for code in our ecosystem. Putting both BIP and
code under the "OR" avoids any confusion about which part is licensed
under which terms and also avoids any hassle when reorganizing, e.g.,
when moving code out of the BIP Markdown file to a separate file etc.

But I don't want this PR to recommend a license, so let me sell this
change as an editorial change to an example, which is warranted because
the MIT is much more known than FSFAP, in particular in this ecosystem.

Author: real-or-random

bip-0003.md

@@ -395,10 +395,10 @@ Copyright section, possibly providing further details on licensing.
 
 For example, a preamble might include the following License header:
 
-    License: CC0-1.0 OR FSFAP
+    License: CC0-1.0 OR MIT
 
 In this case, the BIP (including all auxiliary files) is made available under the terms of both Creative Commons CC0 1.0 Universal as well as the
-FSF All Permissive License, and anyone may modify and redistribute it provided they comply with the terms of
+MIT License, and anyone may modify and redistribute it provided they comply with the terms of
 *either* license, at their option. In other words, the license list is an "OR choice", not an "AND also" requirement. See the [SPDX
 documentation](https://spdx.dev/ids/) and the [SPDX License List](https://spdx.org/licenses/) for further details.