Fix link and add Chapter 15 link

Author: javiervargas

01_0_Introduction.md

@@ -43,7 +43,7 @@ If you have different levels of skill or want to learn different things, you mig
 * If you just want to read about using programming languages, jump to [Chapter Sixteen: Talking to Bitcoin](16_0_Talking_to_Bitcoind.md).
 * If you conversely don't want to do any programming, definitely skip chapters 15-17 while you're reading, and perhaps skip chapters 9-13. The rest of the course should still make sense without them.
 * If you are only interested in Lightning, zap over to [Chapter Nighteen: Understanding Your Lightning Setup](19_0_Understanding_Your_Lightning_Setup.md).
-* If you want to read the major new content added for v2 of the course (2020), following on v1 (2017), read [§3.5: Understanding the Descriptor](03_5_Understanding_the_Descriptor.md), [§4.6: Creating a SegWit Transaction](04_6_Creating_a_Segwit_Transaction.md), [Chapter 7: Expanding Bitcoin with PSBTs](07_0_Expanding_Bitcoin_Transactions_PSBTs.md), [§9.5: Scripting a P2WPKH](09_5_Scripting_a_P2WPKH.md), [§10.5: Scripting a SegWit Script](10_5_Scripting_a_Segwit_Script.md), [Chapter 14: Using Tor](14_0_Using_Tor.md), [Chapter 15: Using i2p], [Chapter 16: Talking to Bitcoind with C](16_0_Talking_to_Bitcoind.md), [Chapter 17: Programming with Libwally](17_0_Programming_with_Libwally.md), [Chapter 18: Talking to Bitcoind with Other Languages](18_0_Talking_to_Bitcoind_Other.md), [Chapter 19: Understanding ](19_0_Understanding_Your_Lightning_Setup.md), and [Chapter 20: Using Lightning](20_0_Using_Lightning.md).
+* If you want to read the major new content added for v2 of the course (2020), following on v1 (2017), read [§3.5: Understanding the Descriptor](03_5_Understanding_the_Descriptor.md), [§4.6: Creating a SegWit Transaction](04_6_Creating_a_Segwit_Transaction.md), [Chapter 7: Expanding Bitcoin with PSBTs](07_0_Expanding_Bitcoin_Transactions_PSBTs.md), [§9.5: Scripting a P2WPKH](09_5_Scripting_a_P2WPKH.md), [§10.5: Scripting a SegWit Script](10_5_Scripting_a_Segwit_Script.md), [Chapter 14: Using Tor](14_0_Using_Tor.md), [Chapter 15: Using i2p](15.0: Using i2p), [Chapter 16: Talking to Bitcoind with C](16_0_Talking_to_Bitcoind.md), [Chapter 17: Programming with Libwally](17_0_Programming_with_Libwally.md), [Chapter 18: Talking to Bitcoind with Other Languages](18_0_Talking_to_Bitcoind_Other.md), [Chapter 19: Understanding Your Lighting Setup](19_0_Understanding_Your_Lightning_Setup.md), and [Chapter 20: Using Lightning](20_0_Using_Lightning.md).
 
 ## Why to Use this Course